subtree(users/wpcarro): docking briefcase at '24f5a642' r/3226

git-subtree-dir: users/wpcarro git-subtree-mainline: 464bbcb15c09813172c79820bcf526bb10cf4208 git-subtree-split: 24f5a642af3aa1627bbff977f0a101907a02c69f Change-Id: I6105b3762b79126b3488359c95978cadb3efa789
author: Vincent Ambo <mail@tazj.in> 2021-12-13T22·51+0300
committer: Vincent Ambo <mail@tazj.in> 2021-12-13T23·15+0300
commit: 019f8fd2113df4c5247c3969c60fd4f0e08f91f7 (patch)
tree: 76a857f61aa88f62a30e854651e8439db77fd0ea /users/wpcarro/scratch
parent: 464bbcb15c09813172c79820bcf526bb10cf4208 (diff)
parent: 6123e976928ca3d8d93f0b2006b10b5f659eb74d (diff)
221 files changed, 13466 insertions, 0 deletions
diff --git a/users/wpcarro/scratch/README.md b/users/wpcarro/scratch/README.md
new file mode 100644
index 0000000000..8259ac70d9
--- /dev/null
+++ b/users/wpcarro/scratch/README.md
@@ -0,0 +1,6 @@
+# Scratch
+
+The purpose of the `scratch` directory is to host practice exercises. Practice
+encompasses things like working on data structures and algorithms problems for
+upcoming coding interviews or general aptitude as well as writing code snippets
+to help me learn a new programming language or understand an unfamiliar concept.
diff --git a/users/wpcarro/scratch/advent-of-code-2019/README.md b/users/wpcarro/scratch/advent-of-code-2019/README.md
new file mode 100644
index 0000000000..e7c105a7f6
--- /dev/null
+++ b/users/wpcarro/scratch/advent-of-code-2019/README.md
@@ -0,0 +1,4 @@
+# 2019 Advent of Code
+
+Here are my attempts at the 2019 Advent of Code challenge before my dedication
+to the effort plummeted.
diff --git a/users/wpcarro/scratch/advent-of-code-2019/day_1.py b/users/wpcarro/scratch/advent-of-code-2019/day_1.py
new file mode 100644
index 0000000000..bd4024e3ec
--- /dev/null
+++ b/users/wpcarro/scratch/advent-of-code-2019/day_1.py
@@ -0,0 +1,119 @@
+from math import floor
+
+xs = [
+    102473,
+    84495,
+    98490,
+    68860,
+    62204,
+    72810,
+    65185,
+    145951,
+    77892,
+    108861,
+    70764,
+    67286,
+    74002,
+    80773,
+    52442,
+    131505,
+    107162,
+    126993,
+    59784,
+    64231,
+    91564,
+    68585,
+    98735,
+    69020,
+    77332,
+    60445,
+    65826,
+    111506,
+    95431,
+    146687,
+    135119,
+    86804,
+    95915,
+    85434,
+    111303,
+    148127,
+    132921,
+    136213,
+    89004,
+    143137,
+    144853,
+    143017,
+    104386,
+    100612,
+    54760,
+    63813,
+    144191,
+    84481,
+    69718,
+    84936,
+    98621,
+    124993,
+    92736,
+    60369,
+    137284,
+    101902,
+    112726,
+    51784,
+    126496,
+    85005,
+    101661,
+    137278,
+    136637,
+    90340,
+    100209,
+    53683,
+    50222,
+    132060,
+    98797,
+    139054,
+    135638,
+    100632,
+    137849,
+    125333,
+    103981,
+    76954,
+    134352,
+    74229,
+    93402,
+    62552,
+    50286,
+    57066,
+    98439,
+    120708,
+    117827,
+    107884,
+    72837,
+    148663,
+    125645,
+    61460,
+    120555,
+    142473,
+    106668,
+    58612,
+    58576,
+    143366,
+    90058,
+    121087,
+    89546,
+    126161,
+]
+
+
+def fuel_for_mass(x):
+    """Return the amount of fuel (in mass) required for a mass of X. The total
+    amount of fuel includes the amount of fuel required for the fuel itself,
+    since fuel also has a mass weights."""
+    mass_fuel = floor(x / 3) - 2
+    if mass_fuel < 0:
+        return 0
+    else:
+        fuel_fuel = fuel_for_mass(mass_fuel)
+        return mass_fuel + fuel_fuel
+
+
+print(sum(fuel_for_mass(x) for x in xs))
diff --git a/users/wpcarro/scratch/advent-of-code-2019/day_2.py b/users/wpcarro/scratch/advent-of-code-2019/day_2.py
new file mode 100644
index 0000000000..77774c1bb5
--- /dev/null
+++ b/users/wpcarro/scratch/advent-of-code-2019/day_2.py
@@ -0,0 +1,32 @@
+from itertools import product
+
+x = [
+    1, 0, 0, 3, 1, 1, 2, 3, 1, 3, 4, 3, 1, 5, 0, 3, 2, 1, 10, 19, 1, 6, 19, 23,
+    2, 23, 6, 27, 2, 6, 27, 31, 2, 13, 31, 35, 1, 10, 35, 39, 2, 39, 13, 43, 1,
+    43, 13, 47, 1, 6, 47, 51, 1, 10, 51, 55, 2, 55, 6, 59, 1, 5, 59, 63, 2, 9,
+    63, 67, 1, 6, 67, 71, 2, 9, 71, 75, 1, 6, 75, 79, 2, 79, 13, 83, 1, 83, 10,
+    87, 1, 13, 87, 91, 1, 91, 10, 95, 2, 9, 95, 99, 1, 5, 99, 103, 2, 10, 103,
+    107, 1, 107, 2, 111, 1, 111, 5, 0, 99, 2, 14, 0, 0
+]
+
+
+def interpret(i, x):
+    op, a, b, out = x[i + 0], x[i + 1], x[i + 2], x[i + 3]
+    if op == 1:
+        x[out] = x[a] + x[b]
+        return interpret(i + 4, x)
+    elif op == 2:
+        x[out] = x[a] * x[b]
+        return interpret(i + 4, x)
+    elif op == 99:
+        return x
+    else:
+        raise Exception('Unsupported opcode: {}. {}, {}'.format(op, a, b))
+
+
+for a, b in product(range(100), range(100)):
+    y = x[:]
+    y[1] = a
+    y[2] = b
+    if interpret(0, y)[0] == 19690720:
+        print(100 * a + b)
diff --git a/users/wpcarro/scratch/advent-of-code-2019/day_3.py b/users/wpcarro/scratch/advent-of-code-2019/day_3.py
new file mode 100644
index 0000000000..6dd863528c
--- /dev/null
+++ b/users/wpcarro/scratch/advent-of-code-2019/day_3.py
@@ -0,0 +1,137 @@
+from math import floor
+from heapq import heappush, heappop
+
+xs = [
+    "R1009", "U993", "L383", "D725", "R163", "D312", "R339", "U650", "R558",
+    "U384", "R329", "D61", "L172", "D555", "R160", "D972", "L550", "D801",
+    "L965", "U818", "L123", "D530", "R176", "D353", "L25", "U694", "L339",
+    "U600", "L681", "D37", "R149", "D742", "R762", "U869", "R826", "U300",
+    "L949", "U978", "L303", "U361", "R136", "D343", "L909", "U551", "R745",
+    "U913", "L566", "D292", "R820", "U886", "R205", "D431", "L93", "D71",
+    "R577", "U872", "L705", "U510", "L698", "U963", "R607", "U527", "L669",
+    "D543", "R690", "U954", "L929", "D218", "R490", "U500", "L589", "D332",
+    "R949", "D538", "R696", "U659", "L188", "U468", "L939", "U833", "L445",
+    "D430", "R78", "D303", "R130", "D649", "R849", "D712", "L511", "U745",
+    "R51", "U973", "R799", "U829", "R605", "D771", "L837", "U204", "L414",
+    "D427", "R538", "U116", "R540", "D168", "R493", "U900", "L679", "U431",
+    "L521", "D500", "L428", "U332", "L954", "U717", "L853", "D339", "L88",
+    "U807", "L607", "D496", "L163", "U468", "L25", "U267", "L759", "D898",
+    "L591", "U445", "L469", "U531", "R596", "D486", "L728", "D677", "R350",
+    "D429", "R39", "U568", "R92", "D875", "L835", "D841", "R877", "U178",
+    "L221", "U88", "R592", "U692", "R455", "U693", "L419", "U90", "R609",
+    "U672", "L293", "U168", "R175", "D456", "R319", "D570", "R504", "D165",
+    "L232", "D624", "L604", "D68", "R807", "D59", "R320", "D281", "L371",
+    "U956", "L788", "D897", "L231", "D829", "R287", "D798", "L443", "U194",
+    "R513", "D925", "L232", "U225", "L919", "U563", "R448", "D889", "R661",
+    "U852", "L950", "D558", "L269", "U186", "L625", "U673", "L995", "U732",
+    "R435", "U849", "L413", "D690", "L158", "D234", "R361", "D458", "L271",
+    "U90", "L781", "U754", "R256", "U162", "L842", "U927", "L144", "D62",
+    "R928", "D238", "R473", "U97", "L745", "U303", "L487", "D349", "L520",
+    "D31", "L825", "U385", "L133", "D948", "L39", "U62", "R801", "D664",
+    "L333", "U134", "R692", "U385", "L658", "U202", "L279", "D374", "R489",
+    "D686", "L182", "U222", "R733", "U177", "R94", "D603", "L376", "U901",
+    "R216", "D851", "L155", "D214", "L460", "U758", "R121", "D746", "L180",
+    "U175", "L943", "U146", "L166", "D251", "L238", "U168", "L642", "D341",
+    "R281", "U182", "R539", "D416", "R553", "D67", "L748", "U272", "R257",
+    "D869", "L340", "U180", "R791", "U138", "L755", "D976", "R731", "U713",
+    "R602", "D284", "L258", "U176", "R509", "U46", "R935", "U576", "R96",
+    "U89", "L913", "U703", "R833"
+]
+ys = [
+    "L1006", "D998", "R94", "D841", "R911", "D381", "R532", "U836", "L299",
+    "U237", "R781", "D597", "L399", "D800", "L775", "D405", "L485", "U636",
+    "R589", "D942", "L878", "D779", "L751", "U711", "L973", "U410", "L151",
+    "U15", "L685", "U417", "L106", "D648", "L105", "D461", "R448", "D743",
+    "L589", "D430", "R883", "U37", "R155", "U350", "L421", "U23", "R337",
+    "U816", "R384", "D671", "R615", "D410", "L910", "U914", "L579", "U385",
+    "R916", "U13", "R268", "D519", "R289", "U410", "L389", "D885", "L894",
+    "U734", "L474", "U707", "L72", "U155", "L237", "U760", "L127", "U806",
+    "L15", "U381", "L557", "D727", "L569", "U320", "L985", "D452", "L8",
+    "D884", "R356", "U732", "L672", "D458", "L485", "U402", "L238", "D30",
+    "R644", "U125", "R753", "U183", "L773", "U487", "R849", "U210", "L164",
+    "D808", "L595", "D668", "L340", "U785", "R313", "D72", "L76", "D263",
+    "R689", "U604", "R471", "U688", "R462", "D915", "R106", "D335", "R869",
+    "U499", "R190", "D916", "R468", "D882", "R56", "D858", "L143", "D741",
+    "L386", "U856", "R50", "U853", "R151", "D114", "L773", "U854", "L290",
+    "D344", "L23", "U796", "L531", "D932", "R314", "U960", "R643", "D303",
+    "L661", "D493", "L82", "D491", "L722", "U848", "L686", "U4", "L985",
+    "D509", "L135", "D452", "R500", "U105", "L326", "D101", "R222", "D944",
+    "L645", "D362", "L628", "U305", "L965", "U356", "L358", "D137", "R787",
+    "U728", "R967", "U404", "R18", "D928", "L695", "D965", "R281", "D597",
+    "L791", "U731", "R746", "U163", "L780", "U41", "L255", "U81", "L530",
+    "D964", "R921", "D297", "R475", "U663", "L226", "U623", "L984", "U943",
+    "L143", "U201", "R926", "U572", "R343", "U839", "R764", "U751", "R128",
+    "U939", "R987", "D108", "R474", "U599", "R412", "D248", "R125", "U797",
+    "L91", "D761", "L840", "U290", "L281", "U779", "R650", "D797", "R185",
+    "D320", "L25", "U378", "L696", "U332", "R75", "D620", "L213", "D667",
+    "R558", "U267", "L846", "U306", "R939", "D220", "R311", "U827", "R345",
+    "U534", "R56", "D679", "R48", "D845", "R898", "U8", "R862", "D960", "R753",
+    "U319", "L886", "D795", "R805", "D265", "R876", "U729", "R894", "D368",
+    "R858", "U744", "R506", "D327", "L903", "U919", "L721", "U507", "L463",
+    "U753", "R775", "D719", "R315", "U128", "R17", "D376", "R999", "D386",
+    "L259", "U181", "L162", "U605", "L265", "D430", "R35", "D968", "R207",
+    "U466", "R796", "D667", "R93", "U749", "L315", "D410", "R312", "U929",
+    "L923", "U260", "R638"
+]
+
+
+def to_coords(xs):
+    row, col = 0, 0
+    coords = []
+    for x in xs:
+        d, amt = x[0], int(x[1:])
+        if d == 'U':
+            for i in range(1, amt + 1):
+                coords.append((row + i, col))
+            row += amt
+        elif d == 'D':
+            for i in range(1, amt + 1):
+                coords.append((row - i, col))
+            row -= amt
+        elif d == 'L':
+            for i in range(1, amt + 1):
+                coords.append((row, col - i))
+            col -= amt
+        elif d == 'R':
+            for i in range(1, amt + 1):
+                coords.append((row, col + i))
+            col += i
+    return coords
+
+
+def contains(row, col, d):
+    if row not in d:
+        return False
+    return col in d[row]
+
+
+def intersections(xs, ys):
+    d = {}
+    ints = set()
+    for row, col in to_coords(xs):
+        if row in d:
+            d[row].add(col)
+        else:
+            d[row] = {col}
+    for row, col in to_coords(ys):
+        if contains(row, col, d):
+            ints.add((row, col))
+    return ints
+
+
+def trace_to(coord, xs):
+    count = 0
+    for coord_x in to_coords(xs):
+        count += 1
+        if coord_x == coord:
+            return count
+    raise Exception("Intersection doesn't exist")
+
+
+answer = []
+for coord in intersections(xs, ys):
+    x = trace_to(coord, xs)
+    y = trace_to(coord, ys)
+    heappush(answer, x + y)
+
+print(heappop(answer))
diff --git a/users/wpcarro/scratch/advent-of-code-2019/day_4.py b/users/wpcarro/scratch/advent-of-code-2019/day_4.py
new file mode 100644
index 0000000000..adef73b452
--- /dev/null
+++ b/users/wpcarro/scratch/advent-of-code-2019/day_4.py
@@ -0,0 +1,35 @@
+import re
+
+start = 134792
+end = 675810
+
+
+def satisfies(x):
+    x = str(x)
+    result = False
+    double, not_decreasing = False, False
+
+    # double and *only* double exists
+    for i in range(len(x) - 1):
+        # double and left-of-a  is BOL or !x
+        #        and right-of-b is EOL or !x
+        a, b = x[i], x[i + 1]
+        bol = i - 1 < 0
+        eol = i + 2 >= len(x)
+        if a == b and (bol or x[i - 1] != a) and (eol or x[i + 2] != a):
+            double = True
+            break
+
+    # not_decreasing
+    prev = int(x[0])
+    for a in x[1:]:
+        a = int(a)
+        if prev > a:
+            return False
+        prev = a
+    not_decreasing = True
+
+    return double and not_decreasing
+
+
+print(len([x for x in range(start, end + 1) if satisfies(x)]))
diff --git a/users/wpcarro/scratch/advent-of-code-2019/day_5.py b/users/wpcarro/scratch/advent-of-code-2019/day_5.py
new file mode 100644
index 0000000000..3d82846e61
--- /dev/null
+++ b/users/wpcarro/scratch/advent-of-code-2019/day_5.py
@@ -0,0 +1,170 @@
+x = [
+    3, 225, 1, 225, 6, 6, 1100, 1, 238, 225, 104, 0, 1102, 31, 68, 225, 1001,
+    13, 87, 224, 1001, 224, -118, 224, 4, 224, 102, 8, 223, 223, 1001, 224, 7,
+    224, 1, 223, 224, 223, 1, 174, 110, 224, 1001, 224, -46, 224, 4, 224, 102,
+    8, 223, 223, 101, 2, 224, 224, 1, 223, 224, 223, 1101, 13, 60, 224, 101,
+    -73, 224, 224, 4, 224, 102, 8, 223, 223, 101, 6, 224, 224, 1, 224, 223,
+    223, 1101, 87, 72, 225, 101, 47, 84, 224, 101, -119, 224, 224, 4, 224,
+    1002, 223, 8, 223, 1001, 224, 6, 224, 1, 223, 224, 223, 1101, 76, 31, 225,
+    1102, 60, 43, 225, 1102, 45, 31, 225, 1102, 63, 9, 225, 2, 170, 122, 224,
+    1001, 224, -486, 224, 4, 224, 102, 8, 223, 223, 101, 2, 224, 224, 1, 223,
+    224, 223, 1102, 29, 17, 224, 101, -493, 224, 224, 4, 224, 102, 8, 223, 223,
+    101, 1, 224, 224, 1, 223, 224, 223, 1102, 52, 54, 225, 1102, 27, 15, 225,
+    102, 26, 113, 224, 1001, 224, -1560, 224, 4, 224, 102, 8, 223, 223, 101, 7,
+    224, 224, 1, 223, 224, 223, 1002, 117, 81, 224, 101, -3645, 224, 224, 4,
+    224, 1002, 223, 8, 223, 101, 6, 224, 224, 1, 223, 224, 223, 4, 223, 99, 0,
+    0, 0, 677, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1105, 0, 99999, 1105, 227, 247,
+    1105, 1, 99999, 1005, 227, 99999, 1005, 0, 256, 1105, 1, 99999, 1106, 227,
+    99999, 1106, 0, 265, 1105, 1, 99999, 1006, 0, 99999, 1006, 227, 274, 1105,
+    1, 99999, 1105, 1, 280, 1105, 1, 99999, 1, 225, 225, 225, 1101, 294, 0, 0,
+    105, 1, 0, 1105, 1, 99999, 1106, 0, 300, 1105, 1, 99999, 1, 225, 225, 225,
+    1101, 314, 0, 0, 106, 0, 0, 1105, 1, 99999, 8, 226, 677, 224, 102, 2, 223,
+    223, 1005, 224, 329, 1001, 223, 1, 223, 1108, 677, 226, 224, 102, 2, 223,
+    223, 1006, 224, 344, 101, 1, 223, 223, 108, 677, 226, 224, 102, 2, 223,
+    223, 1006, 224, 359, 101, 1, 223, 223, 7, 677, 226, 224, 102, 2, 223, 223,
+    1005, 224, 374, 101, 1, 223, 223, 1007, 226, 677, 224, 102, 2, 223, 223,
+    1005, 224, 389, 101, 1, 223, 223, 8, 677, 677, 224, 102, 2, 223, 223, 1006,
+    224, 404, 1001, 223, 1, 223, 1007, 677, 677, 224, 1002, 223, 2, 223, 1006,
+    224, 419, 101, 1, 223, 223, 1108, 677, 677, 224, 1002, 223, 2, 223, 1005,
+    224, 434, 1001, 223, 1, 223, 1107, 226, 677, 224, 102, 2, 223, 223, 1005,
+    224, 449, 101, 1, 223, 223, 107, 226, 226, 224, 102, 2, 223, 223, 1006,
+    224, 464, 101, 1, 223, 223, 1108, 226, 677, 224, 1002, 223, 2, 223, 1005,
+    224, 479, 1001, 223, 1, 223, 7, 677, 677, 224, 102, 2, 223, 223, 1006, 224,
+    494, 1001, 223, 1, 223, 1107, 677, 226, 224, 102, 2, 223, 223, 1005, 224,
+    509, 101, 1, 223, 223, 107, 677, 677, 224, 1002, 223, 2, 223, 1006, 224,
+    524, 101, 1, 223, 223, 1008, 677, 677, 224, 1002, 223, 2, 223, 1006, 224,
+    539, 101, 1, 223, 223, 7, 226, 677, 224, 1002, 223, 2, 223, 1005, 224, 554,
+    101, 1, 223, 223, 108, 226, 226, 224, 1002, 223, 2, 223, 1006, 224, 569,
+    101, 1, 223, 223, 1008, 226, 677, 224, 102, 2, 223, 223, 1005, 224, 584,
+    101, 1, 223, 223, 8, 677, 226, 224, 1002, 223, 2, 223, 1005, 224, 599, 101,
+    1, 223, 223, 1007, 226, 226, 224, 1002, 223, 2, 223, 1005, 224, 614, 101,
+    1, 223, 223, 1107, 226, 226, 224, 1002, 223, 2, 223, 1006, 224, 629, 101,
+    1, 223, 223, 107, 677, 226, 224, 1002, 223, 2, 223, 1005, 224, 644, 1001,
+    223, 1, 223, 1008, 226, 226, 224, 1002, 223, 2, 223, 1006, 224, 659, 101,
+    1, 223, 223, 108, 677, 677, 224, 1002, 223, 2, 223, 1005, 224, 674, 1001,
+    223, 1, 223, 4, 223, 99, 226
+]
+
+# Interpretter spec:
+# Op-code width: 2
+# ABCDE
+# A:  Mode of 3rd parameter
+# B:  Mode of 2rd parameter
+# C:  Mode of 1st parameter
+# DE: 2-digit op-code
+#
+# Not every op-code has the same arity.
+#
+# Parameter modes:
+# - positional: index of memory. 0
+# - immediate: raw value. 1
+# Assert that you never attempt to write to an "immediate value"
+
+# Parameter modes
+POS = '0'  # positional parameter mode
+VAL = '1'  # immediate parameter mode
+
+
+# Pasted from day-2.py
+# interpretter :: Int -> [Int] -> [Int] -> IO ()
+def interpret(i, x, argv=[], outs=[]):
+    """Values in `argv` will be applied to any `input` fields."""
+    # The widest op-code we'll see is 3 + 2 = 5 for either addition or
+    # multiplication since each of those is a 3-arity function with a two-digit
+    # op-code.
+    instruction = '{:05d}'.format(x[i])
+    op = instruction[-2:]
+
+    if op == '01':
+        a, b, out = x[i + 1], x[i + 2], x[i + 3]
+        mode_a, mode_b, mode_out = instruction[2], instruction[1], instruction[
+            0]
+        a = a if mode_a == VAL else x[a]
+        b = b if mode_b == VAL else x[b]
+        assert mode_out == POS
+        x[out] = a + b
+        return interpret(i + 4, x, argv=argv, outs=outs)
+    elif op == '02':
+        a, b, out = x[i + 1], x[i + 2], x[i + 3]
+        mode_a, mode_b, mode_out = instruction[2], instruction[1], instruction[
+            0]
+        a = a if mode_a == VAL else x[a]
+        b = b if mode_b == VAL else x[b]
+        assert mode_out == POS
+        x[out] = a * b
+        return interpret(i + 4, x, argv=argv, outs=outs)
+    # input
+    elif op == '03':
+        a = x[i + 1]
+        mode_a = instruction[2]
+        assert mode_a == POS
+        # What's the pythonic way to defensively get this value?
+        if len(argv) and argv[0] is not None:
+            x[a] = argv[0]
+            return interpret(i + 2, x, argv=argv[1:], outs=outs)
+        elif len(outs) and outs[-1] is not None:
+            x[a] = outs[-1]
+            return interpret(i + 2, x, argv=argv, outs=outs)
+        else:
+            # Here we want to block until the user applies input. This could be
+            # done easily with message passing for something similar.
+            x[a] = int(input('Enter: '))
+            return interpret(i + 2, x, argv=argv)
+    # output
+    elif op == '04':
+        a = x[i + 1]
+        mode_a = instruction[2]
+        a = a if mode_a == VAL else x[a]
+        outs.append(a)
+        return interpret(i + 2, x, argv=argv, outs=outs)
+    # jump-if-true
+    elif op == '05':
+        a, b = x[i + 1], x[i + 2]
+        mode_a, mode_b = instruction[2], instruction[1]
+        a = a if mode_a == VAL else x[a]
+        b = b if mode_b == VAL else x[b]
+        if a != 0:
+            return interpret(b, x, argv=argv, outs=outs)
+        else:
+            return interpret(i + 3, x, argv=argv, outs=outs)
+    # jump-if-false
+    elif op == '06':
+        a, b = x[i + 1], x[i + 2]
+        mode_a, mode_b = instruction[2], instruction[1]
+        a = a if mode_a == VAL else x[a]
+        b = b if mode_b == VAL else x[b]
+        if a == 0:
+            return interpret(b, x, argv=argv, outs=outs)
+        else:
+            return interpret(i + 3, x, argv=argv, outs=outs)
+        pass
+    # less than
+    elif op == '07':
+        a, b, out = x[i + 1], x[i + 2], x[i + 3]
+        mode_a, mode_b, mode_out = instruction[2], instruction[1], instruction[
+            0]
+        a = a if mode_a == VAL else x[a]
+        b = b if mode_b == VAL else x[b]
+        assert mode_out == POS
+        if a < b:
+            x[out] = 1
+        else:
+            x[out] = 0
+        return interpret(i + 4, x, argv=argv, outs=outs)
+    # equals
+    elif op == '08':
+        a, b, out = x[i + 1], x[i + 2], x[i + 3]
+        mode_a, mode_b, mode_out = instruction[2], instruction[1], instruction[
+            0]
+        a = a if mode_a == VAL else x[a]
+        b = b if mode_b == VAL else x[b]
+        assert mode_out == POS
+        if a == b:
+            x[out] = 1
+        else:
+            x[out] = 0
+        return interpret(i + 4, x, argv=argv, outs=outs)
+    elif op == '99':
+        return x[0]
+    else:
+        raise Exception('Unsupported opcode: {}.'.format(op))
diff --git a/users/wpcarro/scratch/advent-of-code-2019/day_6.py b/users/wpcarro/scratch/advent-of-code-2019/day_6.py
new file mode 100644
index 0000000000..aba99b8239
--- /dev/null
+++ b/users/wpcarro/scratch/advent-of-code-2019/day_6.py
@@ -0,0 +1,155 @@
+from graphviz import Digraph
+
+data = """6WF)DRK 2PT)PSM H42)FN8 1XR)LQD HRK)9KL TD6)H8W 98Z)BJM RCQ)LVG
+RWQ)Q7H 2PS)X94 NHB)25X PXC)W57 L8L)MVX CFK)D8K R1B)43T PDY)QKX FQK)82K JJ6)MQJ
+FB6)6V1 R28)5MZ BN2)5HN 6BQ)JVC W57)22C MQJ)DL2 MTC)84R RH8)CRN Y27)3GN CKQ)31C
+R7V)9BK ZDY)PDY X2Q)Y6S Q8B)SAN 1Z3)PVT R87)57R KCJ)44X PWQ)9CB HLC)VYW HFP)9XS
+X33)MC3 RYS)R7R JRF)VHW 79R)FXZ YQQ)STV 8J6)JWX Q6D)RV6 LL9)B4D 6R1)T1Z VK9)42M
+PQP)17N K6C)HMK GLY)N47 KDW)CDC DQ4)RY5 SND)FDR 7YF)1VN MDT)B3S D3F)98Z 5VH)MR7
+KNR)2L8 CJW)QDL FWY)14X SJD)79R COM)BXW T2B)FPB B2Q)BRJ Z21)HYC VHW)5XR WZ4)2JM
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+DBK)FB6 45T)HLT L11)JVN HB5)K6C QH5)888 BTJ)J55 8BT)8ZS FR1)XGL S87)PS9 C4K)BN2
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+FPB)HMN SC4)57D 5LQ)R2T LXM)R8Z JQ6)G4B WNL)GK2 42M)P75 LM3)YPK ZN6)753 PN4)835
+C4H)JY1 LR4)VD5 PSM)P1W VWL)C6C G2V)WBC 85M)R24 B1V)QW7 175)2PM Y1V)1ZH 34W)3MJ
+WN7)TTB 3PV)CQD N7Y)9T3 223)8D4 RV6)LJ9 HFP)JRF VMT)DNB GJP)D3F J5G)KMS 7Q6)ZW2
+YCB)JBY XGN)MNL 888)DSP X61)Q6N WT5)X12 SDN)FD1 2QC)54W V98)964 T7S)YVZ MLX)9VZ
+FR8)QH5 TVQ)2PS 2PV)FHY F4S)MPT 3J9)JNB J6M)GDC Q4C)MJN 9VZ)BZK P2P)B69 WBC)M1W
+D97)HPF JKB)9L4 593)6YJ RMB)4Q5 QZB)38C H12)6R1 MKY)DDD HGX)CRG P53)WY7 22B)GMM
+44X)2D8 DT8)L7H 3Y2)D3S FB8)68N 3BC)1XR 4XF)TVQ VPL)R7V Z4V)JSK B3S)FW5 49Z)YQQ
+99V)D13 54Q)SS7 CYC)TXH PQ3)78W X4M)G9H WFL)M99 ZYY)3Y2 12Y)PSW W38)P29 H8W)JJ6
+P66)VPH GK2)45T H5F)FJT JDJ)SNV 14F)96Q JG6)TQ4 2L6)52Q SCY)CBJ 3GN)KNC KLM)XPR
+DH1)QZB DMB)X7G DPL)7SX D97)N3N GNS)T95 53P)GW2 BHR)HNB YHX)XQV 2CR)Y1V C9D)Z7P
+FN8)2PT 6LF)FCQ JNL)LQR SPV)YCB HGX)N83 VS4)8BT 5RH)FTX HYC)X2J 69V)J6S 9XS)PN4
+SD7)5Q3 2RN)82D QRY)FFY K2Y)3X2 79Z)S2Z YN2)Y64 JKB)MDT KJ8)NDH N57)5VH 3XK)1Q1
+SCH)FJ6 17N)GMP QR4)7V2 GLV)GLY NHF)ZDY QDL)S14 QF1)BMC ZLF)DHN 3JF)7TR MKS)GCY
+964)91R 9L4)L5G RRX)6ZB CD7)73M 3X2)PGC HNB)S9Z L94)KLM 8MQ)SCR 18C)3TJ M4Y)BTJ
+BC9)5YR TV5)SCY 2NX)8CC C9Z)MTC B69)3QP HR7)CHJ 8ZS)JRN 31C)TJW D43)4NH 93Q)X9X
+T95)DNZ LQ5)BC9 9T5)S2C RP8)DH1 GCY)SD7 Y44)9B5 VG5)ZYY 7RY)V3L PWV)Q4L NF1)7YF
+DRK)Y8V D13)GYG TW1)2PB ZVZ)2VV BRJ)V2V 9CB)Y7B MK4)9CJ TMR)6XS HWF)GK8 QTF)S1S
+DFW)6LF N3S)WN1 N2Q)MSW CZ5)X61 FXZ)C4H SCQ)MF7 9LY)3LN 5MZ)PMX CN9)WF9 FHY)PR8
+S38)NWH M29)G5S 4NH)GZJ 5YR)54H CLX)MNY TJD)HQL RRZ)4GQ YHB)CZ5 P37)93Q YJG)3Q3
+95B)QMF CMQ)BLZ QD9)45M JSK)R28 YCW)CLX 8K3)JGB N8M)PQW P75)1HL XBS)T2T 22C)PVW
+689)6MS FFY)RWX YHL)2G8 Y8V)4P1 Y7B)62Z YKJ)JDJ 1HL)5LQ PZ3)B1C 52Q)7HB 3Q2)ZV7
+YBF)Z4V J95)SDH NM6)YBF 8YN)J3M J6S)KNR PVT)N4X SDH)RFW RFW)7Y1 JCB)52B 3MJ)H58
+4QF)XHZ F62)DFW 7LJ)KDW JHL)C9D B4D)Q8B 342)YGS PFR)ZQT Z9K)TNS 8F8)WLB 94N)DMB
+QBT)RYS 3VR)KRR 8D4)ST6 X9N)2PV 632)8K3 MX5)XNP 57D)Y27 18D)PQP D3F)RJ4 PLS)PBL
+1JP)YDC 79V)BG2 S14)2NX 4Q5)NCQ FTX)555 2PM)KMH HQC)RMB 9Z9)BNZ XHV)Y94 7ZP)YHR
+BNZ)49Z W6D)LX6 SLS)JL3 PVW)P9W Z1L)HB5 DS5)G2V Z9Q)RV8 DFR)LPJ 836)693 K94)VWL
+HRG)836 J3V)593 52N)LPK 9KL)Y7M LX6)F7D JL3)511 L4G)D97 1RH)Y9F NJ2)LML GW2)9WV
+8KZ)NRC XQV)G6D R8Z)QF7 326)HML R7R)8PM 622)YCW WQY)LGS NF1)FF3 5LQ)QF1 5XR)PTD
+V2V)PFR 9T5)JQ6 CBQ)8KZ VZ4)HVQ TJW)DQT 9WT)5M6 CFK)YHL JR9)1JP Y1K)CF4 8WS)JPY
+VYC)1D6 GKK)7J9 JTG)RRX 6V1)F74 1H5)QR4 SN3)NMG MF7)GQ1 RYK)SCH BNZ)9LY 1DJ)9LP
+L6W)5BK FCQ)BFL DCJ)3RD MXD)8MQ RWX)1RH NBF)WKM K6C)WNH H58)L6W Y7B)BJH PGC)NBF
+96Q)Q2W F7D)BSN 223)Z9K K94)VYC X9X)7M3 Q1M)3J9 QXF)XQ6 DD7)3Q2 Q1L)NHB 79T)LXQ
+8TZ)M29 21T)Q4C B1C)NSF 8D8)FJ2 LJH)HGJ QS2)PS1 5KX)Z2L C6C)6BQ VQ2)2YP P87)N8M
+ST5)L4G 8SP)W5M T4H)69V 9WF)GHS FF3)SND C5G)GKK VQ2)X4M P43)8J6 TD6)384 66V)CN9
+CX4)T9T NCQ)2JQ 29K)K8K RY5)K4Q GQ3)T4H FNH)P32 3BC)PRQ 5HN)4QY M1W)BGT 84R)ST5
+S45)CJW CK4)W7G SGX)19R S2C)7ZX DHN)W5Y 8D9)HM2 BSB)SPV D8K)DFV JHL)2L6 KYP)12Y
+KDN)6X7 Y44)SQZ 6G6)SJD N7D)QGF Q84)8WJ F89)LL9 LYJ)2RN 25X)Q84 HM3)53P JNB)QD9
+SLW)1DQ 384)3BC PR8)NGV 49N)7ZP 65H)LHJ 6XS)S45 ZMG)FR1 X2M)Y86 QD3)QLM P4R)PQ3
+RTK)4M3 4YW)N7D R7V)M4M 73M)CBF DFV)64R Z7P)LMK HRG)Y1K 3ZM)BCZ WY7)QXP DMC)9Q5
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+8HP)4G7 KDN)YYL LHJ)SDN P6P)XMC W5Y)RYK HX8)KW3 Z2L)H12 WPF)T2B L7H)BGW MNL)17B
+GHS)66V QKX)XWV FW5)W38 PDK)Y34 FKG)Q6D DQT)YJG 15G)79V 4VK)51Y BJH)LR4 48V)6GC
+DM5)Y1F CM5)VG5 KB8)HRK 5HN)RCQ 6JP)SDQ LGH)NJ2 L94)N7Y 4Y2)ZLF 25G)C4K K8K)SLS
+232)ZVZ GQ1)58J RV8)H5F 78W)565 YCF)8D9 DZG)99V N83)CKR TN2)ZCX NGV)8SP BSN)FTN
+LPJ)94N 3Q3)Q1M JVX)971 54W)LGH 67Y)P66 R24)P37 3QP)QTY YHR)FLT GMP)NM6 NDH)632
+PWV)8D8 LMK)3PV ZWJ)KB8 967)4VK 3B1)WN7 XWS)5CV YR1)FNH 565)4PH 5BK)V98 W5Y)FR8
+PS1)HX8 38C)XXG XWV)1YC M4M)LQ5 S9Z)49N XMC)R1B YYL)VC9 GMM)SCQ LXQ)J95 51Y)RP8
+HLT)XBS 82K)B8Z NR5)7K3 K2M)67Y SF6)W6D CF4)85M MC3)LXM HMN)RNM BFL)4XF MT2)PM4
+VWK)JKB 3JF)ZTZ QWJ)9QQ KRR)TJD VYW)Z9Q CK4)QS2 8NQ)NR5 57R)BHR 8WM)YHB Y86)GNS
+2Y2)Z21 X12)9QX LJ9)YKJ 3RD)8F1 7SQ)CK4 ZXH)3XK DDD)5KX ZCX)PYR GZJ)KXL KC5)52N
+PM4)RYP 14X)ZWJ FJ6)175 17B)689 HQL)14F LQR)DBK LGS)4Y2 2QQ)SGR 2VV)8F8 J6S)LM3
+RTP)YZ5 XDD)14K VQ4)MT2 KMH)KYC CKR)RTP VD5)MRM CM5)KRW BG3)XDD PGM)J4T MY3)JVX
+Z8F)WNP BKM)WT5 FLT)KTF N7D)8M1 Y19)CMQ HPF)WDL 65H)JJP 2MQ)66S 4Q5)54Q Q2W)ZL4
+QTY)659 MRM)9Z9 X2J)SC4 YWH)RB3 FTN)LYJ LMK)N7M SGX)15G KW3)FQK 3VV)JNL JWX)R8R
+9Z3)9MB BMC)N3S W7G)Z1L SD7)MW2 376)RH8 NWT)JHL 7CD)N2Z KTR)HM3 1Q1)TDZ DY9)2CR
+6YJ)14G FWT)JDW C2S)C5G SNV)J6M 5TK)YWH J3M)8HF HM2)GJP P9W)7CD 1VN)SGX KMS)RBK
+64R)B1V 62D)3VV 61D)F4S XPR)SKN FJT)N3P 9WV)D43 TQ8)BDB 46H)K4V 8WJ)MXD NDM)9WF
+8ZL)1QJ SCR)2MQ 7Y9)LJH VPH)MKY YDC)PDK 4G7)65H 2JM)NYY T9T)VMT 8M1)TSK G5S)X4N
+6FH)KYP D98)DQW G6D)C2S 6X7)N2Q 1QJ)T7S ZL4)J8T 5BT)3VR 835)KCJ YM8)3RG Y7M)PWQ
+54W)9W4 CBF)7LJ 4T5)8WS RHQ)HBK CQD)D98 HGJ)J6R JVC)79Z FD1)PKF VC9)5BT C4H)6WF
+D3S)P6P MR7)BG3 R6V)DF3 9X1)NQ5 ZTZ)2Y2 8WM)HFP CDC)376 TQ4)M4Y 9MB)N1R HBK)DQ4
+1DQ)CYC WNP)DM8 CBJ)LK7 ZT8)FWY LQD)PNN 555)9Z3 TNS)D9L QMF)L11 FR8)5RH WF9)R87
+NKG)5HT L5G)91W N2Z)YV9 9B5)CD7 ZV7)8NQ ST6)74T ZJ6)CQV S18)47M 74T)8YN WNH)TN2
+874)46H 3VV)PZ3 Y1F)42W MPT)2LP FDR)HWF X7G)RTK 52B)P4R RYP)G93 NWH)YCF 7TR)FB8
+RWQ)6FH 8F8)HLC CRN)P2P B6D)KC5 PNN)HRG""".split()
+
+# COM is the root in this tree
+
+
+# parent :: Vertex -> [Edge] -> Maybe(Vertex)
+def parent(x, xs):
+    for a, b in xs:
+        if b == x:
+            return a
+    return None
+
+
+# parents :: Vertex -> [Edge] -> [Vertex]
+def parents(x, xs):
+    parents = []
+    p = parent(x, xs)
+    while p:
+        parents.append(p)
+        p = parent(p, xs)
+    return parents
+
+
+# alias Vertex :: String
+# alias Edge :: (String, String)
+# to_edge_list :: [String] -> [(String, String)]
+def to_edge_list(xs):
+    """Returns a list of tuples where (A, B) represents a directed edge from
+    vertex A to vertex B."""
+    return [(x[0:3], x[4:]) for x in xs]
+
+
+# to_graphviz :: [Edge] -> String
+def to_graphviz(xs):
+    d = Digraph()
+    for a, b in xs:
+        d.node(a, label=a)
+        d.edge(a, b)
+    return d.source
+
+
+graph = to_edge_list(data)
+you = parents('YOU', graph)
+san = parents('SAN', graph)
+
+# Distance from YOU to shared point with SAN
+yd = 1
+for i in range(len(you)):
+    if you[i] in san:
+        break
+    yd += 1
+
+# Distance from SAN to shared point with YOU
+sd = 1
+for i in range(len(san)):
+    if san[i] in you:
+        break
+    sd += 1
+
+print('Number of orbital transfers required: {}'.format(yd - 1 + sd - 1))
diff --git a/users/wpcarro/scratch/advent-of-code-2019/day_7.py b/users/wpcarro/scratch/advent-of-code-2019/day_7.py
new file mode 100644
index 0000000000..14597d5104
--- /dev/null
+++ b/users/wpcarro/scratch/advent-of-code-2019/day_7.py
@@ -0,0 +1,49 @@
+from day_5 import interpret
+from itertools import permutations
+
+# TODO: I may need to re-write this in Elixir modelling each amplifier as a
+# `Process` and `Process.send`ing each amplifier the signals.
+
+data = [
+    3, 8, 1001, 8, 10, 8, 105, 1, 0, 0, 21, 38, 59, 76, 89, 106, 187, 268, 349,
+    430, 99999, 3, 9, 1002, 9, 3, 9, 101, 2, 9, 9, 1002, 9, 4, 9, 4, 9, 99, 3,
+    9, 1001, 9, 5, 9, 1002, 9, 5, 9, 1001, 9, 2, 9, 1002, 9, 3, 9, 4, 9, 99, 3,
+    9, 1001, 9, 4, 9, 102, 4, 9, 9, 1001, 9, 3, 9, 4, 9, 99, 3, 9, 101, 4, 9,
+    9, 1002, 9, 5, 9, 4, 9, 99, 3, 9, 1002, 9, 3, 9, 101, 5, 9, 9, 1002, 9, 3,
+    9, 4, 9, 99, 3, 9, 102, 2, 9, 9, 4, 9, 3, 9, 1002, 9, 2, 9, 4, 9, 3, 9,
+    1002, 9, 2, 9, 4, 9, 3, 9, 101, 2, 9, 9, 4, 9, 3, 9, 1002, 9, 2, 9, 4, 9,
+    3, 9, 102, 2, 9, 9, 4, 9, 3, 9, 101, 1, 9, 9, 4, 9, 3, 9, 1001, 9, 1, 9, 4,
+    9, 3, 9, 1002, 9, 2, 9, 4, 9, 3, 9, 101, 2, 9, 9, 4, 9, 99, 3, 9, 1002, 9,
+    2, 9, 4, 9, 3, 9, 101, 2, 9, 9, 4, 9, 3, 9, 1002, 9, 2, 9, 4, 9, 3, 9, 101,
+    1, 9, 9, 4, 9, 3, 9, 102, 2, 9, 9, 4, 9, 3, 9, 102, 2, 9, 9, 4, 9, 3, 9,
+    101, 2, 9, 9, 4, 9, 3, 9, 101, 2, 9, 9, 4, 9, 3, 9, 102, 2, 9, 9, 4, 9, 3,
+    9, 1001, 9, 2, 9, 4, 9, 99, 3, 9, 1002, 9, 2, 9, 4, 9, 3, 9, 1001, 9, 2, 9,
+    4, 9, 3, 9, 101, 1, 9, 9, 4, 9, 3, 9, 101, 2, 9, 9, 4, 9, 3, 9, 101, 2, 9,
+    9, 4, 9, 3, 9, 102, 2, 9, 9, 4, 9, 3, 9, 1001, 9, 2, 9, 4, 9, 3, 9, 102, 2,
+    9, 9, 4, 9, 3, 9, 1001, 9, 1, 9, 4, 9, 3, 9, 1001, 9, 2, 9, 4, 9, 99, 3, 9,
+    1001, 9, 2, 9, 4, 9, 3, 9, 102, 2, 9, 9, 4, 9, 3, 9, 1001, 9, 2, 9, 4, 9,
+    3, 9, 102, 2, 9, 9, 4, 9, 3, 9, 101, 2, 9, 9, 4, 9, 3, 9, 1002, 9, 2, 9, 4,
+    9, 3, 9, 1002, 9, 2, 9, 4, 9, 3, 9, 1002, 9, 2, 9, 4, 9, 3, 9, 101, 1, 9,
+    9, 4, 9, 3, 9, 101, 1, 9, 9, 4, 9, 99, 3, 9, 101, 2, 9, 9, 4, 9, 3, 9, 102,
+    2, 9, 9, 4, 9, 3, 9, 1002, 9, 2, 9, 4, 9, 3, 9, 1001, 9, 2, 9, 4, 9, 3, 9,
+    1001, 9, 2, 9, 4, 9, 3, 9, 1001, 9, 2, 9, 4, 9, 3, 9, 1001, 9, 1, 9, 4, 9,
+    3, 9, 1001, 9, 2, 9, 4, 9, 3, 9, 1001, 9, 2, 9, 4, 9, 3, 9, 102, 2, 9, 9,
+    4, 9, 99
+]
+
+data_a, data_b, data_c, data_d, data_e = data[:], data[:], data[:], data[:], data[:]
+
+# m = 0
+# for a, b, c, d, e in permutations(range(5, 10)):
+#     answer = None
+#     z = 0
+#     while z is not None:
+#         print(a, b, c, d, e)
+#         print('---')
+#         v = interpret(0, data_a, argv=[a, z])
+#         print(v)
+#         w = interpret(0, data_b, argv=[b, v])
+#         x = interpret(0, data_c, argv=[c, w])
+#         y = interpret(0, data_d, argv=[d, x])
+#         z = interpret(0, data_e, argv=[e, y])
+#         m = max(m, z)
diff --git a/users/wpcarro/scratch/crack_the_coding_interview/11_1.py b/users/wpcarro/scratch/crack_the_coding_interview/11_1.py
new file mode 100644
index 0000000000..ec7b65dae0
--- /dev/null
+++ b/users/wpcarro/scratch/crack_the_coding_interview/11_1.py
@@ -0,0 +1,40 @@
+# Implementation for a problem from "Crack the Coding Interview".
+#
+# Dependencies:
+# - python 2.7.16
+# - entr 4.1
+#
+# To run the tests, run: `python 11_1.py`
+# For a tight development loop, run: `echo 11_1.py | entr python /_`
+#
+# Author: William Carroll <wpcarro@gmail.com>
+
+################################################################################
+# Implementation
+################################################################################
+def insert_sorted(xs, ys):
+    """
+    Merges `ys` into `xs` and ensures that the result is sorted.
+
+    Assumptions:
+    - `xs` and `ys` are both sorted.
+    - `xs` has enough unused space to accommodate each element in `ys`.
+    """
+    for y in ys:
+        xi = xs.index(None) - 1
+        yi = xs.index(None)
+        xs[yi] = y
+        while xi != -1 and y < xs[xi]:
+            xs[xi], xs[yi] = xs[yi], xs[xi]
+            xi, yi = xi - 1, yi - 1
+    return xs
+
+################################################################################
+# Tests
+################################################################################
+assert insert_sorted([1, 3, 5, None, None], [2, 4]) == [1, 2, 3, 4, 5]
+assert insert_sorted([None, None], [2, 4]) == [2, 4]
+assert insert_sorted([None, None], [2, 4]) == [2, 4]
+assert insert_sorted([1, 1, None, None], [0, 0]) == [0, 0, 1, 1]
+assert insert_sorted([1, 1, None, None], [1, 1]) == [1, 1, 1, 1]
+print('All tests pass!')
diff --git a/users/wpcarro/scratch/crack_the_coding_interview/to_tree.hs b/users/wpcarro/scratch/crack_the_coding_interview/to_tree.hs
new file mode 100644
index 0000000000..8496d88c0c
--- /dev/null
+++ b/users/wpcarro/scratch/crack_the_coding_interview/to_tree.hs
@@ -0,0 +1,11 @@
+data Tree a = Node a [Tree a] deriving (Show)
+
+withRoot :: [a] -> [Tree a]
+withRoot xs = xs |> toThing |> fmap buildTree
+
+buildTree :: (a, [a])
+
+
+toTree :: [a] -> Tree a
+toTree [x]      = Node x []
+toTree [x | xs] = Node x (toTree xs)
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/array-traversals.py b/users/wpcarro/scratch/data_structures_and_algorithms/array-traversals.py
new file mode 100644
index 0000000000..35cb439281
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/array-traversals.py
@@ -0,0 +1,87 @@
+# This is practice for various types of list traversals that turn up.
+
+xs = range(10)
+n = len(xs)
+
+print('---')
+# pythonic left-to-right traversal
+result = ''
+for x in xs:
+    result += str(x)
+print(result)
+
+print('---')
+# left-to-right traversal
+result = ''
+for i in range(n):
+    result += str(xs[i])
+print(result)
+
+print('---')
+# right-to-left traversal
+result = ''
+for i in range(n):
+    result += str(xs[n - 1 - i])
+print(result)
+
+print('---')
+# 2x left-to-right traversal
+result = ''
+for i in range(2 * n):
+    result += str(xs[i % n])
+print(result)
+
+print('---')
+# 2x right-to-left traversal
+result = ''
+for i in range(2 * n):
+    result += str(xs[(n - 1 - i) % n])
+print(result)
+
+################################################################################
+# Table traversals
+################################################################################
+
+table = [[row * 10 + i for i in range(10)] for row in range(3)]
+row_ct = len(table)
+col_ct = len(table[0])
+
+print('---')
+# 3x10 table traversal
+result = ''
+for row in table:
+    r = ''
+    for col in row:
+        r += '{:3d}'.format(col)
+    result += r + '\n'
+print(result[0:-1])
+
+print('---')
+# 3x10 table traversal
+result = ''
+for row in range(row_ct):
+    r = ''
+    for col in range(col_ct):
+        r += '{:3d}'.format(table[row][col])
+    result += r + '\n'
+print(result[0:-1])
+
+print('---')
+# 3x10 table traversal (reverse)
+result = ''
+for row in range(row_ct):
+    r = ''
+    for col in range(col_ct):
+        r += '{:3d}'.format(table[row_ct - 1 - row][col_ct - 1 - col])
+    result += r + '\n'
+print(result)
+
+print('---')
+# 3x10 column-row traversal
+result = ''
+for col in range(col_ct):
+    r = ''
+    for row in range(row_ct):
+        r += '{:3d}'.format(table[row][col])
+    result += r + '\n'
+print(result)
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/balanced-binary-tree.py b/users/wpcarro/scratch/data_structures_and_algorithms/balanced-binary-tree.py
new file mode 100644
index 0000000000..01fd965fd5
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/balanced-binary-tree.py
@@ -0,0 +1,145 @@
+import unittest
+from itertools import combinations
+
+
+def balanced(xs):
+    """Return True if `xs` contains no two values that differ by more than
+    one."""
+    if len(xs) == 0 or len(xs) == 1:
+        return True
+    if len(xs) == 2:
+        return math.abs(xs[0] - xs[1]) <= 1
+    else:
+        pass
+
+
+def is_leaf(node):
+    return node.left is None and node.right is None
+
+
+def is_balanced(tree_root):
+    """Returns True if the difference between the depths of any two leaf nodes
+    does not exceed 1."""
+    depths = set()
+    populate_depths(tree_root, 0, depths)
+
+    # cartesian product - only the top half
+    for diff in set(abs(a - b) for a, b in combinations(depths, 2)):
+        if diff > 1:
+            return False
+
+    return True
+
+
+def populate_depths(node, depth, depths):
+    if is_leaf(node):
+        depths.add(depth)
+    else:
+        if node.left is not None:
+            populate_depths(node.left, depth + 1, depths)
+        if node.right is not None:
+            populate_depths(node.right, depth + 1, depths)
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    class BinaryTreeNode(object):
+        def __init__(self, value):
+            self.value = value
+            self.left = None
+            self.right = None
+
+        def insert_left(self, value):
+            self.left = Test.BinaryTreeNode(value)
+            return self.left
+
+        def insert_right(self, value):
+            self.right = Test.BinaryTreeNode(value)
+            return self.right
+
+    def test_full_tree(self):
+        tree = Test.BinaryTreeNode(5)
+        left = tree.insert_left(8)
+        right = tree.insert_right(6)
+        left.insert_left(1)
+        left.insert_right(2)
+        right.insert_left(3)
+        right.insert_right(4)
+        result = is_balanced(tree)
+        self.assertTrue(result)
+
+    def test_both_leaves_at_the_same_depth(self):
+        tree = Test.BinaryTreeNode(3)
+        left = tree.insert_left(4)
+        right = tree.insert_right(2)
+        left.insert_left(1)
+        right.insert_right(9)
+        result = is_balanced(tree)
+        self.assertTrue(result)
+
+    def test_leaf_heights_differ_by_one(self):
+        tree = Test.BinaryTreeNode(6)
+        left = tree.insert_left(1)
+        right = tree.insert_right(0)
+        right.insert_right(7)
+        result = is_balanced(tree)
+        self.assertTrue(result)
+
+    def test_leaf_heights_differ_by_two(self):
+        tree = Test.BinaryTreeNode(6)
+        left = tree.insert_left(1)
+        right = tree.insert_right(0)
+        right_right = right.insert_right(7)
+        right_right.insert_right(8)
+        result = is_balanced(tree)
+        self.assertFalse(result)
+
+    def test_three_leaves_total(self):
+        tree = Test.BinaryTreeNode(1)
+        left = tree.insert_left(5)
+        right = tree.insert_right(9)
+        right.insert_left(8)
+        right.insert_right(5)
+        result = is_balanced(tree)
+        self.assertTrue(result)
+
+    def test_both_subtrees_superbalanced(self):
+        tree = Test.BinaryTreeNode(1)
+        left = tree.insert_left(5)
+        right = tree.insert_right(9)
+        right_left = right.insert_left(8)
+        right.insert_right(5)
+        right_left.insert_left(7)
+        result = is_balanced(tree)
+        self.assertFalse(result)
+
+    def test_both_subtrees_superbalanced_two(self):
+        tree = Test.BinaryTreeNode(1)
+        left = tree.insert_left(2)
+        right = tree.insert_right(4)
+        left.insert_left(3)
+        left_right = left.insert_right(7)
+        left_right.insert_right(8)
+        right_right = right.insert_right(5)
+        right_right_right = right_right.insert_right(6)
+        right_right_right.insert_right(9)
+        result = is_balanced(tree)
+        self.assertFalse(result)
+
+    def test_only_one_node(self):
+        tree = Test.BinaryTreeNode(1)
+        result = is_balanced(tree)
+        self.assertTrue(result)
+
+    def test_linked_list_tree(self):
+        tree = Test.BinaryTreeNode(1)
+        right = tree.insert_right(2)
+        right_right = right.insert_right(3)
+        right_right.insert_right(4)
+        result = is_balanced(tree)
+        self.assertTrue(result)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/bit-manipulation.py b/users/wpcarro/scratch/data_structures_and_algorithms/bit-manipulation.py
new file mode 100644
index 0000000000..dc30bb5088
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/bit-manipulation.py
@@ -0,0 +1,32 @@
+def test(x, i):
+    return x & (1 << i) != 0
+
+
+def set(x, i):
+    return x | (1 << i)
+
+
+def clear(x, i):
+    return x & ~(1 << i)
+
+
+def toggle(x, i):
+    if test(x, i):
+        return clear(x, i)
+    else:
+        return set(x, i)
+
+
+def test_single(x):
+    if x == 0:
+        return False
+    else:
+        return x & (x - 1) == 0
+
+
+print(test(0b1010, 3))
+print('{0:b}'.format(set(0b1010, 1)))
+print('{0:b}'.format(clear(0b1010, 1)))
+print('{0:b}'.format(toggle(0b1010, 2)))
+print(test_single(0b1010))
+print(test_single(0b1000))
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/bracket-validator.py b/users/wpcarro/scratch/data_structures_and_algorithms/bracket-validator.py
new file mode 100644
index 0000000000..a50f8b074e
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/bracket-validator.py
@@ -0,0 +1,63 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+# is_valid :: String -> Boolean
+def is_valid(xs):
+    s = []
+    seeking = {
+        '}': '{',
+        ']': '[',
+        ')': '(',
+    }
+    openers = seeking.values()
+    closers = seeking.keys()
+    for c in xs:
+        if c in openers:
+            s.append(c)
+        elif c in closers:
+            if not s:
+                return False
+            elif s[-1] != seeking.get(c):
+                return False
+            else:
+                s.pop()
+    return len(s) == 0
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_valid_short_code(self):
+        result = is_valid('()')
+        self.assertTrue(result)
+
+    def test_valid_longer_code(self):
+        result = is_valid('([]{[]})[]{{}()}')
+        self.assertTrue(result)
+
+    def test_interleaved_openers_and_closers(self):
+        result = is_valid('([)]')
+        self.assertFalse(result)
+
+    def test_mismatched_opener_and_closer(self):
+        result = is_valid('([][]}')
+        self.assertFalse(result)
+
+    def test_missing_closer(self):
+        result = is_valid('[[]()')
+        self.assertFalse(result)
+
+    def test_extra_closer(self):
+        result = is_valid('[[]]())')
+        self.assertFalse(result)
+
+    def test_empty_string(self):
+        result = is_valid('')
+        self.assertTrue(result)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/bst-checker.py b/users/wpcarro/scratch/data_structures_and_algorithms/bst-checker.py
new file mode 100644
index 0000000000..689be97a85
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/bst-checker.py
@@ -0,0 +1,121 @@
+import unittest
+
+
+################################################################################
+# Implementation
+################################################################################
+# is_leaf :: Node(a) -> Boolean
+def is_leaf(node):
+    return not node.left and not node.right
+
+
+# is_binary_search_tree :: Node(Integer) -> Set(Int) -> Set(Int) -> Boolean
+def is_binary_search_tree_a(node, la=set(), ra=set()):
+    """My first solution for this problem."""
+    for x in la:
+        if not node.value < x:
+            return False
+    for x in ra:
+        if not node.value > x:
+            return False
+    if is_leaf(node):
+        return True
+    elif not node.left:
+        return is_binary_search_tree(
+            node.right,
+            la=la,
+            ra=ra ^ {node.value},
+        )
+    elif not node.right:
+        return is_binary_search_tree(node.left, la=la ^ {node.value}, ra=ra)
+    else:
+        return all([
+            is_binary_search_tree(node.left, la=la ^ {node.value}, ra=ra),
+            is_binary_search_tree(node.right, la=la, ra=ra ^ {node.value})
+        ])
+
+
+# is_binary_search_tree :: Node(Int) -> Maybe(Int) -> Maybe(Int) -> Boolean
+def is_binary_search_tree(node, lb=None, ub=None):
+    if lb:
+        if node.value < lb:
+            return False
+    if ub:
+        if node.value > ub:
+            return False
+    if is_leaf(node):
+        return True
+    elif not node.right:
+        return is_binary_search_tree(node.left, lb=lb, ub=node.value)
+    elif not node.left:
+        return is_binary_search_tree(node.right, lb=node.value, ub=ub)
+    else:
+        return is_binary_search_tree(
+            node.left, lb=lb, ub=node.value) and is_binary_search_tree(
+                node.right, lb=node.value, ub=ub)
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    class BinaryTreeNode(object):
+        def __init__(self, value):
+            self.value = value
+            self.left = None
+            self.right = None
+
+        def insert_left(self, value):
+            self.left = Test.BinaryTreeNode(value)
+            return self.left
+
+        def insert_right(self, value):
+            self.right = Test.BinaryTreeNode(value)
+            return self.right
+
+    def test_valid_full_tree(self):
+        tree = Test.BinaryTreeNode(50)
+        left = tree.insert_left(30)
+        right = tree.insert_right(70)
+        left.insert_left(10)
+        left.insert_right(40)
+        right.insert_left(60)
+        right.insert_right(80)
+        result = is_binary_search_tree(tree)
+        self.assertTrue(result)
+
+    def test_both_subtrees_valid(self):
+        tree = Test.BinaryTreeNode(50)
+        left = tree.insert_left(30)
+        right = tree.insert_right(80)
+        left.insert_left(20)
+        left.insert_right(60)
+        right.insert_left(70)
+        right.insert_right(90)
+        result = is_binary_search_tree(tree)
+        self.assertFalse(result)
+
+    def test_descending_linked_list(self):
+        tree = Test.BinaryTreeNode(50)
+        left = tree.insert_left(40)
+        left_left = left.insert_left(30)
+        left_left_left = left_left.insert_left(20)
+        left_left_left.insert_left(10)
+        result = is_binary_search_tree(tree)
+        self.assertTrue(result)
+
+    def test_out_of_order_linked_list(self):
+        tree = Test.BinaryTreeNode(50)
+        right = tree.insert_right(70)
+        right_right = right.insert_right(60)
+        right_right.insert_right(80)
+        result = is_binary_search_tree(tree)
+        self.assertFalse(result)
+
+    def test_one_node_tree(self):
+        tree = Test.BinaryTreeNode(50)
+        result = is_binary_search_tree(tree)
+        self.assertTrue(result)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/cafe-order-checker.py b/users/wpcarro/scratch/data_structures_and_algorithms/cafe-order-checker.py
new file mode 100644
index 0000000000..e34a2b136a
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/cafe-order-checker.py
@@ -0,0 +1,91 @@
+import unittest
+
+
+################################################################################
+# Implementation
+################################################################################
+def is_first_come_first_served(to, di, xs):
+    # All the guards, assertions we should need.
+    if to == di == xs == []:
+        return True
+    elif to == di == []:
+        return False
+    elif to == []:
+        return di == xs
+    elif to == []:
+        return di == xs
+    elif di == []:
+        return to == xs
+    elif xs == []:
+        return False
+    elif len(xs) != (len(to) + len(di)):
+        return False
+
+    fst, snd = to, di
+
+    if xs[0] == to[0]:
+        fst, snd = to, di
+    elif xs[0] == di[0]:
+        fst, snd = di, to
+    else:
+        return False
+
+    fst_done, snd_done = False, False
+    fi, si = 1, 0
+
+    for i in range(1, len(xs)):
+        # Short-circuit and avoid index-out-of-bounds without introducing overly
+        # defensive, sloppy code.
+        if fst_done:
+            return snd[si:] == xs[i:]
+        elif snd_done:
+            return fst[fi:] == xs[i:]
+
+        if fst[fi] == xs[i]:
+            fi += 1
+        elif snd[si] == xs[i]:
+            si += 1
+        else:
+            return False
+
+        fst_done, snd_done = fi == len(fst), si == len(snd)
+
+    return True
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_both_registers_have_same_number_of_orders(self):
+        result = is_first_come_first_served([1, 4, 5], [2, 3, 6],
+                                            [1, 2, 3, 4, 5, 6])
+        self.assertTrue(result)
+
+    def test_registers_have_different_lengths(self):
+        result = is_first_come_first_served([1, 5], [2, 3, 6], [1, 2, 6, 3, 5])
+        self.assertFalse(result)
+
+    def test_one_register_is_empty(self):
+        result = is_first_come_first_served([], [2, 3, 6], [2, 3, 6])
+        self.assertTrue(result)
+
+    def test_served_orders_is_missing_orders(self):
+        result = is_first_come_first_served([1, 5], [2, 3, 6], [1, 6, 3, 5])
+        self.assertFalse(result)
+
+    def test_served_orders_has_extra_orders(self):
+        result = is_first_come_first_served([1, 5], [2, 3, 6],
+                                            [1, 2, 3, 5, 6, 8])
+        self.assertFalse(result)
+
+    def test_one_register_has_extra_orders(self):
+        result = is_first_come_first_served([1, 9], [7, 8], [1, 7, 8])
+        self.assertFalse(result)
+
+    def test_one_register_has_unserved_orders(self):
+        result = is_first_come_first_served([55, 9], [7, 8], [1, 7, 8, 9])
+        self.assertFalse(result)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/cake-thief.py b/users/wpcarro/scratch/data_structures_and_algorithms/cake-thief.py
new file mode 100644
index 0000000000..9eddb34b2d
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/cake-thief.py
@@ -0,0 +1,71 @@
+import unittest
+from math import floor
+
+
+################################################################################
+# Solution
+################################################################################
+def max_duffel_bag_value(xs, cap):
+    ct = (cap + 1)
+    maxes = [0] * ct
+    for c in range(cap + 1):
+        for w, v in xs:
+            if w == 0 and v > 0:
+                return float('inf')
+            if w == c:
+                maxes[c:] = [max(maxes[c], v)] * (ct - c)
+            elif w < c:
+                d = c - w
+                maxes[c:] = [max(maxes[c], v + maxes[d])] * (ct - c)
+            else:
+                continue
+    return maxes[cap]
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_one_cake(self):
+        actual = max_duffel_bag_value([(2, 1)], 9)
+        expected = 4
+        self.assertEqual(actual, expected)
+
+    def test_two_cakes(self):
+        actual = max_duffel_bag_value([(4, 4), (5, 5)], 9)
+        expected = 9
+        self.assertEqual(actual, expected)
+
+    def test_only_take_less_valuable_cake(self):
+        actual = max_duffel_bag_value([(4, 4), (5, 5)], 12)
+        expected = 12
+        self.assertEqual(actual, expected)
+
+    def test_lots_of_cakes(self):
+        actual = max_duffel_bag_value([(2, 3), (3, 6), (5, 1), (6, 1), (7, 1),
+                                       (8, 1)], 7)
+        expected = 12
+        self.assertEqual(actual, expected)
+
+    def test_value_to_weight_ratio_is_not_optimal(self):
+        actual = max_duffel_bag_value([(51, 52), (50, 50)], 100)
+        expected = 100
+        self.assertEqual(actual, expected)
+
+    def test_zero_capacity(self):
+        actual = max_duffel_bag_value([(1, 2)], 0)
+        expected = 0
+        self.assertEqual(actual, expected)
+
+    def test_cake_with_zero_value_and_weight(self):
+        actual = max_duffel_bag_value([(0, 0), (2, 1)], 7)
+        expected = 3
+        self.assertEqual(actual, expected)
+
+    def test_cake_with_non_zero_value_and_zero_weight(self):
+        actual = max_duffel_bag_value([(0, 5)], 5)
+        expected = float('inf')
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/coins.py b/users/wpcarro/scratch/data_structures_and_algorithms/coins.py
new file mode 100644
index 0000000000..eb5754f982
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/coins.py
@@ -0,0 +1,57 @@
+import unittest
+from math import floor
+
+################################################################################
+# Solution
+################################################################################
+
+# change_possibilities :: Int -> [Int] -> Int
+def change_possibilities(n, xs):
+    combinations = [0] * (n + 1)
+    combinations[0] = 1
+
+    for x in xs:
+        for i in range(len(combinations)):
+            if i >= x:
+                combinations[i] += combinations[i - x]
+
+    return combinations[n]
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+
+    def test_sample_input(self):
+        actual = change_possibilities(4, (1, 2, 3))
+        expected = 4
+        self.assertEqual(actual, expected)
+
+    def test_one_way_to_make_zero_cents(self):
+        actual = change_possibilities(0, (1, 2))
+        expected = 1
+        self.assertEqual(actual, expected)
+
+    def test_no_ways_if_no_coins(self):
+        actual = change_possibilities(1, ())
+        expected = 0
+        self.assertEqual(actual, expected)
+
+    def test_big_coin_value(self):
+        actual = change_possibilities(5, (25, 50))
+        expected = 0
+        self.assertEqual(actual, expected)
+
+    def test_big_target_amount(self):
+        actual = change_possibilities(50, (5, 10))
+        expected = 6
+        self.assertEqual(actual, expected)
+
+    def test_change_for_one_dollar(self):
+        actual = change_possibilities(100, (1, 5, 10, 25, 50))
+        expected = 292
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/conways-game-of-life.py b/users/wpcarro/scratch/data_structures_and_algorithms/conways-game-of-life.py
new file mode 100644
index 0000000000..3836bcd0c6
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/conways-game-of-life.py
@@ -0,0 +1,78 @@
+from itertools import product
+from random import choice
+from time import sleep
+from os import system
+from math import floor
+from colorama import Back, Fore, Style
+
+################################################################################
+# Simulation of Conway's Game of Life. The goal here was to write this with a
+# small amount of code as a proof-of-concept that could be run in the terminal.
+#
+# If you'd like to tinker with the rules, see the conditionals defined in the
+# `advance/1` function. For other parameters, like the board size and refresh
+# rate, refer to the while-loop defined at the bottom of this file.
+################################################################################
+
+
+def init_board(n, init_alive_percentage):
+    """Initialize a board of size `n` by `n`. Supply a percentage,
+    `init_alive_percentage`, representing the number of cells in the board that
+    should be alive from the start."""
+    alive_count = floor(n * init_alive_percentage)
+    distribution = [True] * alive_count + [False] * (n - alive_count)
+    return [[choice(distribution) for _ in range(n)] for _ in range(n)]
+
+
+def neighbors(coord, board):
+    """Return the neighbors for a given `coord` on a `board`."""
+    n = len(board)
+    row, col = coord
+    return [
+        board[(row + row_d) % n][(col + col_d) % n]
+        for row_d, col_d in product([-1, 0, 1], [-1, 0, 1])
+        if (row_d, col_d) != (0, 0)
+    ]
+
+
+def advance(board):
+    """Advance the state of the `board` from T[n] to T[n+1]."""
+    n = len(board)
+    new_board = [[False for _ in range(n)] for _ in range(n)]
+    for row in range(n):
+        for col in range(n):
+            alive_count = len([x for x in neighbors((row, col), board) if x])
+            # Loneliness
+            if alive_count == 0:
+                new_board[row][col] = False
+            # Status Quo
+            elif alive_count == 1:
+                new_board[row][col] = board[row][col]
+            # Cooperation
+            elif alive_count == 2:
+                new_board[row][col] = True
+            # Resource starvation
+            elif alive_count >= 3:
+                new_board[row][col] = False
+    return new_board
+
+
+def print_board(board):
+    """Print the game `board` in a human-readable way."""
+    result = ''
+    for row in board:
+        for col in row:
+            if col:
+                result += Back.GREEN + '1 ' + Style.RESET_ALL
+            else:
+                result += Back.RED + '0 ' + Style.RESET_ALL
+        result += '\n'
+    print(result)
+
+
+board = init_board(100, 0.50)
+while True:
+    system('clear')
+    print_board(board)
+    sleep(0.15)
+    board = advance(board)
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/delete-node.py b/users/wpcarro/scratch/data_structures_and_algorithms/delete-node.py
new file mode 100644
index 0000000000..7e431e2249
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/delete-node.py
@@ -0,0 +1,60 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+def delete_node(x):
+    if not x.next:
+        raise Exception('Cannot delete the last node in a linked list.')
+    else:
+        x.value = x.next.value
+        x.next = x.next.next
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    class LinkedListNode(object):
+        def __init__(self, value, next=None):
+            self.value = value
+            self.next = next
+
+        def get_values(self):
+            node = self
+            values = []
+            while node is not None:
+                values.append(node.value)
+                node = node.next
+            return values
+
+    def setUp(self):
+        self.fourth = Test.LinkedListNode(4)
+        self.third = Test.LinkedListNode(3, self.fourth)
+        self.second = Test.LinkedListNode(2, self.third)
+        self.first = Test.LinkedListNode(1, self.second)
+
+    def test_node_at_beginning(self):
+        delete_node(self.first)
+        actual = self.first.get_values()
+        expected = [2, 3, 4]
+        self.assertEqual(actual, expected)
+
+    def test_node_in_middle(self):
+        delete_node(self.second)
+        actual = self.first.get_values()
+        expected = [1, 3, 4]
+        self.assertEqual(actual, expected)
+
+    def test_node_at_end(self):
+        with self.assertRaises(Exception):
+            delete_node(self.fourth)
+
+    def test_one_node_in_list(self):
+        unique = Test.LinkedListNode(1)
+        with self.assertRaises(Exception):
+            delete_node(unique)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/dft.py b/users/wpcarro/scratch/data_structures_and_algorithms/dft.py
new file mode 100644
index 0000000000..127d48c186
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/dft.py
@@ -0,0 +1,65 @@
+from random import choice
+
+
+class Node(object):
+    def __init__(self, value=None, left=None, right=None):
+        self.value = value
+        self.left = left
+        self.right = left
+
+
+def p(node, indent=0):
+    print(indent * ' ' + '|-' + str(node.value))
+    if node.left is not None:
+        p(node.left, indent=indent + 2)
+    if node.right is not None:
+        p(node.right, indent=indent + 2)
+
+
+# read trees (i.e. traversing, parsing)
+# write trees (i.e. generating, printing)
+def random(d=0):
+    left = None
+    right = None
+
+    if choice([True, False]):
+        left = random(d + 1)
+
+    if choice([True, False]):
+        right = random(d + 1)
+
+    return Node(
+        value=d,
+        left=left,
+        right=right,
+    )
+
+
+################################################################################
+# DFTs can be:
+# - imperative (mutable)
+# - functional (immutable)
+# - iterative
+# - recursive
+################################################################################
+
+
+# Iterative
+def traverse(node, f):
+    stack = [(node, 0)]
+
+    while len(stack):
+        node, depth = stack.pop()
+        f(node, depth)
+        print(depth)
+
+        if node.left is not None:
+            stack.append((node.left, depth + 1))
+        if node.right is not None:
+            stack.append((node.right, depth + 1))
+
+
+print('----------------------------------------------------------------------')
+for _ in range(10):
+    traverse(random(), lambda _, d: print(d))
+print()
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/dijkstra-shortest-path.py b/users/wpcarro/scratch/data_structures_and_algorithms/dijkstra-shortest-path.py
new file mode 100644
index 0000000000..03907f6040
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/dijkstra-shortest-path.py
@@ -0,0 +1,48 @@
+from collections import deque
+from heapq import heappush, heappop
+from fixtures import weighted_graph
+
+
+def put(t, x, xs):
+    if t == 'stack':
+        return xs.append(x)
+    if t == 'queue':
+        return xs.append(x)
+    if t == 'priority':
+        return heappush(xs, x)
+
+
+def pop(t, xs):
+    if t == 'stack':
+        return xs.pop()
+    if t == 'queue':
+        return xs.popleft()
+    if t == 'priority':
+        return heappop(xs)
+
+
+# shortest_path :: Vertex -> Vertex -> Graph -> [Vertex]
+def shortest_path(a, b, g):
+    """Returns the shortest path from vertex a to vertex b in graph g."""
+    t = 'priority'
+    xs = []
+    seen = set()
+    # Map(Weight, [Vertex])
+    m = {}
+
+    put(t, (0, [a], a), xs)
+
+    while xs:
+        w0, path, v = pop(t, xs)
+
+        seen.add(v)
+        if v == b:
+            m[w0] = path
+        for w1, x in g.get(v):
+            if x not in seen:
+                put(t, (w0 + w1, path + [x], x), xs)
+
+    return m
+
+
+print(shortest_path('a', 'f', graph_a))
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/find-duplicate-optimize-for-space-beast.py b/users/wpcarro/scratch/data_structures_and_algorithms/find-duplicate-optimize-for-space-beast.py
new file mode 100644
index 0000000000..93fdd9eed2
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/find-duplicate-optimize-for-space-beast.py
@@ -0,0 +1,56 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+def find_duplicate(xs):
+    self_ref_count = 0
+    for i in range(len(xs)):
+        if xs[i] == i + 1:
+            self_ref_count += 1
+    hops = len(xs) - 1 - self_ref_count
+    current = xs[-1]
+    while hops > 0:
+        current = xs[current - 1]
+        hops -= 1
+    return current
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    # TODO: Debug why this fails.
+    def test_darren_from_interview_cake(self):
+        actual = find_duplicate([4, 1, 8, 3, 2, 7, 6, 5, 4])
+        expected = 4
+        self.assertEqual(actual, expected)
+
+    def test_just_the_repeated_number(self):
+        actual = find_duplicate([1, 1])
+        expected = 1
+        self.assertEqual(actual, expected)
+
+    def test_short_list(self):
+        actual = find_duplicate([1, 2, 3, 2])
+        expected = 2
+        self.assertEqual(actual, expected)
+
+    def test_last_cycle(self):
+        actual = find_duplicate([3, 4, 2, 3, 1, 5])
+        expected = 3
+        self.assertEqual(actual, expected)
+
+    def test_medium_list(self):
+        actual = find_duplicate([1, 2, 5, 5, 5, 5])
+        expected = 5
+        self.assertEqual(actual, expected)
+
+    def test_long_list(self):
+        actual = find_duplicate([4, 1, 4, 8, 3, 2, 7, 6, 5])
+        expected = 4
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/find-duplicate-optimize-for-space.py b/users/wpcarro/scratch/data_structures_and_algorithms/find-duplicate-optimize-for-space.py
new file mode 100644
index 0000000000..e2739f0f60
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/find-duplicate-optimize-for-space.py
@@ -0,0 +1,61 @@
+from math import floor
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+def bounds(r):
+    ct = len(r)
+    if ct % 2 == 0:
+        h = int(ct / 2)
+        return ct, h
+    else:
+        h = floor(ct / 2)
+        return ct, h
+
+
+def find_repeat(xs):
+    ct, h = bounds(xs)
+    rl = range(1, h + 1)
+    rr = range(h + 1, ct)
+    while True:
+        nl = len([None for x in xs if x in rl])
+        nr = len([None for x in xs if x in rr])
+        branch = rl if nl > nr else rr
+        if len(branch) == 1:
+            return branch[0]
+        ct, h = bounds(branch)
+        rl = range(branch[0], branch[0])
+        rr = range(branch[0] + h, branch[-1] + 1)
+    raise Exception(
+        'We could not find any duplicates in xs. Perhaps xs did not adhere to the usage contract.'
+    )
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_just_the_repeated_number(self):
+        actual = find_repeat([1, 1])
+        expected = 1
+        self.assertEqual(actual, expected)
+
+    def test_short_list(self):
+        actual = find_repeat([1, 2, 3, 2])
+        expected = 2
+        self.assertEqual(actual, expected)
+
+    def test_medium_list(self):
+        actual = find_repeat([1, 2, 5, 5, 5, 5])
+        expected = 5
+        self.assertEqual(actual, expected)
+
+    def test_long_list(self):
+        actual = find_repeat([4, 1, 4, 8, 3, 2, 7, 6, 5])
+        expected = 4
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/find-rotation-point.py b/users/wpcarro/scratch/data_structures_and_algorithms/find-rotation-point.py
new file mode 100644
index 0000000000..2103a5b84f
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/find-rotation-point.py
@@ -0,0 +1,59 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+def find_rotation_point(xs):
+    """Usage of `visited` here is a hack, but works for the test cases
+    (gulp)."""
+    i = 0
+    j = round(len(xs) / 2)
+    result = None
+    visited = set()
+    while not result:
+        if i in visited:
+            i += 1
+        if j in visited:
+            j -= 1
+        visited.add(i)
+        visited.add(j)
+        if xs[j - 1] > xs[j]:
+            result = j
+        elif xs[i] < xs[j]:
+            i = j
+            j += round((len(xs) - j) / 2)
+        elif xs[i] >= xs[j]:
+            i = j
+            j -= round((j - i) / 2)
+    return result
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_small_list(self):
+        actual = find_rotation_point(['cape', 'cake'])
+        expected = 1
+        self.assertEqual(actual, expected)
+
+    def test_medium_list(self):
+        actual = find_rotation_point(
+            ['grape', 'orange', 'plum', 'radish', 'apple'])
+        expected = 4
+        self.assertEqual(actual, expected)
+
+    def test_large_list(self):
+        actual = find_rotation_point([
+            'ptolemaic', 'retrograde', 'supplant', 'undulate', 'xenoepist',
+            'asymptote', 'babka', 'banoffee', 'engender', 'karpatka',
+            'othellolagkage'
+        ])
+        expected = 5
+        self.assertEqual(actual, expected)
+
+    # Are we missing any edge cases?
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/find-unique-int-among-duplicates.py b/users/wpcarro/scratch/data_structures_and_algorithms/find-unique-int-among-duplicates.py
new file mode 100644
index 0000000000..dfa5de42cc
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/find-unique-int-among-duplicates.py
@@ -0,0 +1,45 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+def find_unique_delivery_id(xs):
+    a = 0
+    for x in xs:
+        a ^= x
+    return a
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_one_drone(self):
+        actual = find_unique_delivery_id([1])
+        expected = 1
+        self.assertEqual(actual, expected)
+
+    def test_unique_id_comes_first(self):
+        actual = find_unique_delivery_id([1, 2, 2])
+        expected = 1
+        self.assertEqual(actual, expected)
+
+    def test_unique_id_comes_last(self):
+        actual = find_unique_delivery_id([3, 3, 2, 2, 1])
+        expected = 1
+        self.assertEqual(actual, expected)
+
+    def test_unique_id_in_middle(self):
+        actual = find_unique_delivery_id([3, 2, 1, 2, 3])
+        expected = 1
+        self.assertEqual(actual, expected)
+
+    def test_many_drones(self):
+        actual = find_unique_delivery_id(
+            [2, 5, 4, 8, 6, 3, 1, 4, 2, 3, 6, 5, 1])
+        expected = 8
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/fixtures.py b/users/wpcarro/scratch/data_structures_and_algorithms/fixtures.py
new file mode 100644
index 0000000000..27689ca76d
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/fixtures.py
@@ -0,0 +1,110 @@
+# Using this module to store commonly used, but annoying to create, data
+# structures for my test inputs.
+#
+# Use like:
+# from fixtures import graph_a
+
+################################################################################
+# Constants
+################################################################################
+
+edge_list = [
+    ('a', 'b'),
+    ('a', 'c'),
+    ('a', 'e'),
+    ('b', 'c'),
+    ('b', 'd'),
+    ('c', 'e'),
+    ('d', 'f'),
+    ('e', 'd'),
+    ('e', 'f'),
+]
+
+unweighted_graph = {
+    'a': {'b', 'c', 'e'},
+    'b': {'c', 'd'},
+    'c': {'e'},
+    'd': {'f'},
+    'e': {'d', 'f'},
+    'f': set(),
+}
+
+adjacencies = {
+    'a': {
+        'a': False,
+        'b': False
+    },
+    'a': [],
+    'a': [],
+    'a': [],
+    'a': [],
+    'a': [],
+    'a': [],
+}
+
+weighted_graph = {
+    'a': {(4, 'b'), (2, 'c'), (4, 'e')},
+    'b': {(5, 'c'), (10, 'd')},
+    'c': {(3, 'e')},
+    'd': {(11, 'f')},
+    'e': {(4, 'd'), (5, 'f')},
+    'f': set(),
+}
+
+# This is `weighted_graph` with each of its weighted edges "expanded".
+expanded_weights_graph = {
+    'a': ['b-1', 'c-1', 'e-1'],
+    'b-1': ['b-2'],
+    'b-2': ['b-3'],
+    'b-3': ['b'],
+    'c-1': ['c'],
+    'e-1': ['e-2'],
+    'e-2': ['e-3'],
+    'e-3': ['e'],
+    # and so on...
+}
+
+unweighted_digraph = {
+    '5': {'2', '0'},
+    '4': {'0', '1'},
+    '3': {'1'},
+    '2': {'3'},
+    '1': set(),
+    '0': set(),
+}
+
+################################################################################
+# Functions
+################################################################################
+
+
+def vertices(xs):
+    result = set()
+    for a, b in xs:
+        result.add(a)
+        result.add(b)
+    return result
+
+
+def edges_to_neighbors(xs):
+    result = {v: set() for v in vertices(xs)}
+    for a, b in xs:
+        result[a].add(b)
+    return result
+
+
+def neighbors_to_edges(xs):
+    result = []
+    for k, ys in xs.items():
+        for y in ys:
+            result.append((k, y))
+    return result
+
+
+def edges_to_adjacencies(xs):
+    return xs
+
+
+# Skipping handling adjacencies because I cannot think of a reasonable use-case
+# for it when the vertex labels are items other than integers. I can think of
+# ways of handling this, but none excite me.
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/graph-coloring.py b/users/wpcarro/scratch/data_structures_and_algorithms/graph-coloring.py
new file mode 100644
index 0000000000..bc7f7ceea5
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/graph-coloring.py
@@ -0,0 +1,180 @@
+import unittest
+from collections import deque
+
+
+################################################################################
+# Solution
+################################################################################
+class GraphNode:
+    def __init__(self, label):
+        self.label = label
+        self.neighbors = set()
+        self.color = None
+
+
+# color_graph :: G(V, E) -> Set(Color) -> IO ()
+def color_graph(graph, colors):
+    q = deque()
+    seen = set()
+    q.append(graph[0])
+
+    while q:
+        node = q.popleft()
+
+        illegal = {n.color for n in node.neighbors}
+        for x in colors:
+            if x not in illegal:
+                node.color = x
+
+        seen.add(node)
+
+        for x in node.neighbors:
+            if x not in seen:
+                q.append(x)
+
+        # TODO: Is this the best way to traverse separate graphs?
+        for x in graph:
+            if x not in seen:
+                q.append(x)
+
+    return 0
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def setUp(self):
+        self.colors = frozenset([
+            'red',
+            'green',
+            'blue',
+            'orange',
+            'yellow',
+            'white',
+        ])
+
+    def assertGraphColoring(self, graph, colors):
+        self.assertGraphHasColors(graph, colors)
+        self.assertGraphColorLimit(graph)
+        for node in graph:
+            self.assertNodeUniqueColor(node)
+
+    def assertGraphHasColors(self, graph, colors):
+        for node in graph:
+            msg = 'Node %r color %r not in %r' % (node.label, node.color,
+                                                  colors)
+            self.assertIn(node.color, colors, msg=msg)
+
+    def assertGraphColorLimit(self, graph):
+        max_degree = 0
+        colors_found = set()
+        for node in graph:
+            degree = len(node.neighbors)
+            max_degree = max(degree, max_degree)
+            colors_found.add(node.color)
+        max_colors = max_degree + 1
+        used_colors = len(colors_found)
+        msg = 'Used %d colors and expected %d at most' % (used_colors,
+                                                          max_colors)
+        self.assertLessEqual(used_colors, max_colors, msg=msg)
+
+    def assertNodeUniqueColor(self, node):
+        for adjacent in node.neighbors:
+            msg = 'Adjacent nodes %r and %r have the same color %r' % (
+                node.label,
+                adjacent.label,
+                node.color,
+            )
+            self.assertNotEqual(node.color, adjacent.color, msg=msg)
+
+    def test_line_graph(self):
+        node_a = GraphNode('a')
+        node_b = GraphNode('b')
+        node_c = GraphNode('c')
+        node_d = GraphNode('d')
+
+        node_a.neighbors.add(node_b)
+        node_b.neighbors.add(node_a)
+        node_b.neighbors.add(node_c)
+        node_c.neighbors.add(node_b)
+        node_c.neighbors.add(node_d)
+        node_d.neighbors.add(node_c)
+
+        graph = [node_a, node_b, node_c, node_d]
+        tampered_colors = list(self.colors)
+        color_graph(graph, tampered_colors)
+        self.assertGraphColoring(graph, self.colors)
+
+    def test_separate_graph(self):
+        node_a = GraphNode('a')
+        node_b = GraphNode('b')
+        node_c = GraphNode('c')
+        node_d = GraphNode('d')
+
+        node_a.neighbors.add(node_b)
+        node_b.neighbors.add(node_a)
+        node_c.neighbors.add(node_d)
+        node_d.neighbors.add(node_c)
+
+        graph = [node_a, node_b, node_c, node_d]
+        tampered_colors = list(self.colors)
+        color_graph(graph, tampered_colors)
+        self.assertGraphColoring(graph, self.colors)
+
+    def test_triangle_graph(self):
+        node_a = GraphNode('a')
+        node_b = GraphNode('b')
+        node_c = GraphNode('c')
+
+        node_a.neighbors.add(node_b)
+        node_a.neighbors.add(node_c)
+        node_b.neighbors.add(node_a)
+        node_b.neighbors.add(node_c)
+        node_c.neighbors.add(node_a)
+        node_c.neighbors.add(node_b)
+
+        graph = [node_a, node_b, node_c]
+        tampered_colors = list(self.colors)
+        color_graph(graph, tampered_colors)
+        self.assertGraphColoring(graph, self.colors)
+
+    def test_envelope_graph(self):
+        node_a = GraphNode('a')
+        node_b = GraphNode('b')
+        node_c = GraphNode('c')
+        node_d = GraphNode('d')
+        node_e = GraphNode('e')
+
+        node_a.neighbors.add(node_b)
+        node_a.neighbors.add(node_c)
+        node_b.neighbors.add(node_a)
+        node_b.neighbors.add(node_c)
+        node_b.neighbors.add(node_d)
+        node_b.neighbors.add(node_e)
+        node_c.neighbors.add(node_a)
+        node_c.neighbors.add(node_b)
+        node_c.neighbors.add(node_d)
+        node_c.neighbors.add(node_e)
+        node_d.neighbors.add(node_b)
+        node_d.neighbors.add(node_c)
+        node_d.neighbors.add(node_e)
+        node_e.neighbors.add(node_b)
+        node_e.neighbors.add(node_c)
+        node_e.neighbors.add(node_d)
+
+        graph = [node_a, node_b, node_c, node_d, node_e]
+        tampered_colors = list(self.colors)
+        color_graph(graph, tampered_colors)
+        self.assertGraphColoring(graph, self.colors)
+
+    def test_loop_graph(self):
+        node_a = GraphNode('a')
+        node_a.neighbors.add(node_a)
+        graph = [node_a]
+        tampered_colors = list(self.colors)
+        with self.assertRaises(Exception):
+            color_graph(graph, tampered_colors)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/graph-to-graphviz.py b/users/wpcarro/scratch/data_structures_and_algorithms/graph-to-graphviz.py
new file mode 100644
index 0000000000..0e7e97a20c
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/graph-to-graphviz.py
@@ -0,0 +1,39 @@
+from graphviz import Digraph
+from collections import deque
+from fixtures import weighted_graph
+
+# There are three ways to model a graph:
+# 1. Edge list: [(Vertex, Vertex)]
+# 2. Neighbors table: Map(Vertex, [Vertex])
+# 3. Adjacency matrix: [[Boolean]]
+#
+# The following graph is a neighbors table.
+
+
+# to_graphviz :: Vertex -> Map(Vertex, [(Vertex, Weight)]) -> String
+def to_graphviz(start, g):
+    """Compiles the graph into GraphViz."""
+    d = Digraph()
+    q = deque()
+    seen = set()
+
+    q.append(start)
+
+    while q:
+        v = q.popleft()
+        if v in seen:
+            continue
+        d.node(v, label=v)
+
+        for w, x in g[v]:
+            d.edge(v, x, label=str(w))
+            q.append(x)
+        seen.add(v)
+
+    return d.source
+
+
+with open('/tmp/test.gv', 'w') as f:
+    src = to_graphviz('a', weighted_graph)
+    f.write(src)
+    print('/tmp/test.gv created!')
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/highest-product-of-3.py b/users/wpcarro/scratch/data_structures_and_algorithms/highest-product-of-3.py
new file mode 100644
index 0000000000..889663e058
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/highest-product-of-3.py
@@ -0,0 +1,89 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+# f :: [Int] -> Int
+def highest_product_of_3(xs):
+    """Here we're greedily storing:
+    - current max
+    - largest product of two
+    - largest positive number
+    - second largest positive number
+    - largest negative number
+    """
+    if len(xs) < 3:
+        raise Exception
+
+    cm = None
+    ld = xs[0] * xs[1]
+    l2 = min(xs[0], xs[1])
+    if xs[0] < 0 or xs[1] < 0:
+        ln = min(xs[0], xs[1])
+    else:
+        ln = 1
+    l = max(xs[0], xs[1])
+
+    for x in xs[2:]:
+        if not cm:
+            cm = max(x * ln * l, ld * x, x * l * l2)  # beware
+            ld = max(ld, x * ln, x * l)
+            ln = min(ln, x)
+            l = max(l, x)
+            if x < l:
+                l2 = max(l2, x)
+        else:
+            cm = max(cm, x * ln * l, x * ld, x * l * l2)
+            ld = max(ld, x * ln, x * l)
+            ln = min(ln, x)
+            l = max(l, x)
+            if x < l:
+                l2 = max(l2, x)
+
+    return cm
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_short_list(self):
+        actual = highest_product_of_3([1, 2, 3, 4])
+        expected = 24
+        self.assertEqual(actual, expected)
+
+    def test_longer_list(self):
+        actual = highest_product_of_3([6, 1, 3, 5, 7, 8, 2])
+        expected = 336
+        self.assertEqual(actual, expected)
+
+    def test_list_has_one_negative(self):
+        actual = highest_product_of_3([-5, 4, 8, 2, 3])
+        expected = 96
+        self.assertEqual(actual, expected)
+
+    def test_list_has_two_negatives(self):
+        actual = highest_product_of_3([-10, 1, 3, 2, -10])
+        expected = 300
+        self.assertEqual(actual, expected)
+
+    def test_list_is_all_negatives(self):
+        actual = highest_product_of_3([-5, -1, -3, -2])
+        expected = -6
+        self.assertEqual(actual, expected)
+
+    def test_error_with_empty_list(self):
+        with self.assertRaises(Exception):
+            highest_product_of_3([])
+
+    def test_error_with_one_number(self):
+        with self.assertRaises(Exception):
+            highest_product_of_3([1])
+
+    def test_error_with_two_numbers(self):
+        with self.assertRaises(Exception):
+            highest_product_of_3([1, 1])
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/inflight-entertainment.py b/users/wpcarro/scratch/data_structures_and_algorithms/inflight-entertainment.py
new file mode 100644
index 0000000000..6e17baef37
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/inflight-entertainment.py
@@ -0,0 +1,35 @@
+# possible :: Int -> [Int] -> Bool
+def possible(flight_duration, film_durations):
+    seeking = set()
+
+    for x in film_durations:
+        if x in seeking:
+            return True
+        else:
+            seeking.add(flight_duration - x)
+
+    return False
+
+
+should = [
+    (10, [1, 9, 8, 8, 8]),
+    (10, [1, 9]),
+    (10, [1, 9, 5, 5, 6]),
+    (1, [0.5, 0.5]),
+    (1, [0.5, 0.5]),
+]
+
+for a, b in should:
+    print("Testing: %s %s" % (a, b))
+    assert possible(a, b)
+
+shouldnt = [
+    (10, [1, 10, 1, 2, 1, 12]),
+    (1, [0.25, 0.25, 0.25, 0.25]),
+    (5, [1, 2, 2]),
+]
+for a, b in shouldnt:
+    print("Testing: %s %s" % (a, b))
+    assert not possible(a, b)
+
+print("Tests pass")
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/knapsack-0-1.py b/users/wpcarro/scratch/data_structures_and_algorithms/knapsack-0-1.py
new file mode 100644
index 0000000000..c72d19d4ed
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/knapsack-0-1.py
@@ -0,0 +1,38 @@
+import unittest
+from math import floor
+
+
+def knapify(xs, capacity=None):
+    assert capacity is not None
+    n = len(xs)
+    # For 0/1 Knapsack, we must use a table, since this will encode which values
+    # work for which items. This is cleaner than including a separate data
+    # structure to capture it.
+    maxes = [[0 for x in range(capacity + 1)] for x in range(n + 1)]
+
+    # Build table maxes[][] in bottom up manner
+    for row in range(n + 1):
+        for col in range(capacity + 1):
+            if row == 0 or col == 0:
+                maxes[row][col] = 0
+            elif xs[row - 1][0] <= col:
+                maxes[row][col] = max(
+                    xs[row - 1][1] + maxes[row - 1][col - xs[row - 1][0]],
+                    maxes[row - 1][col])
+            else:
+                maxes[row][col] = maxes[row - 1][col]
+
+    return maxes[-1][capacity]
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_one_cake(self):
+        actual = knapify([(3, 10), (2, 15), (7, 2), (12, 20)], capacity=12)
+        expected = None
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/kth-to-last.py b/users/wpcarro/scratch/data_structures_and_algorithms/kth-to-last.py
new file mode 100644
index 0000000000..8291e54533
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/kth-to-last.py
@@ -0,0 +1,82 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+def length(x):
+    if not x:
+        return 0
+    else:
+        count = 1
+        while x:
+            x = x.next
+            count += 1
+        return count
+
+
+def kth_to_last_node(k, x):
+    hops = length(x) - 1
+    dest = hops - k
+
+    if k == 0:
+        raise Exception("Our God doesn't support this kind of behavior.")
+
+    if dest < 0:
+        raise Exception('Value k to high for list.')
+
+    while dest > 0:
+        x = x.next
+        dest -= 1
+
+    return x
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    class LinkedListNode(object):
+        def __init__(self, value, next=None):
+            self.value = value
+            self.next = next
+
+        def get_values(self):
+            node = self
+            values = []
+            while node is not None:
+                values.append(node.value)
+                node = node.next
+            return values
+
+    def setUp(self):
+        self.fourth = Test.LinkedListNode(4)
+        self.third = Test.LinkedListNode(3, self.fourth)
+        self.second = Test.LinkedListNode(2, self.third)
+        self.first = Test.LinkedListNode(1, self.second)
+
+    def test_first_to_last_node(self):
+        actual = kth_to_last_node(1, self.first)
+        expected = self.fourth
+        self.assertEqual(actual, expected)
+
+    def test_second_to_last_node(self):
+        actual = kth_to_last_node(2, self.first)
+        expected = self.third
+        self.assertEqual(actual, expected)
+
+    def test_first_node(self):
+        actual = kth_to_last_node(4, self.first)
+        expected = self.first
+        self.assertEqual(actual, expected)
+
+    def test_k_greater_than_linked_list_length(self):
+        with self.assertRaises(Exception):
+            kth_to_last_node(5, self.first)
+
+    def test_k_is_zero(self):
+        with self.assertRaises(Exception):
+            kth_to_last_node(0, self.first)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/largest-stack.py b/users/wpcarro/scratch/data_structures_and_algorithms/largest-stack.py
new file mode 100644
index 0000000000..aab9671eb6
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/largest-stack.py
@@ -0,0 +1,107 @@
+import unittest
+
+
+class Stack(object):
+    def __init__(self):
+        """Initialize an empty stack"""
+        self.items = []
+
+    def push(self, item):
+        """Push a new item onto the stack"""
+        self.items.append(item)
+
+    def pop(self):
+        """Remove and return the last item"""
+        # If the stack is empty, return None
+        # (it would also be reasonable to throw an exception)
+        if not self.items:
+            return None
+
+        return self.items.pop()
+
+    def peek(self):
+        """Return the last item without removing it"""
+        if not self.items:
+            return None
+        return self.items[-1]
+
+
+class MaxStack(object):
+    # Implement the push, pop, and get_max methods
+    def __init__(self):
+        self.m = Stack()
+        self.stack = Stack()
+
+    def push(self, item):
+        if self.m.peek() is None:
+            self.m.push(item)
+        elif item >= self.m.peek():
+            self.m.push(item)
+        self.stack.push(item)
+
+    def pop(self):
+        x = self.stack.pop()
+        if x == self.m.peek():
+            self.m.pop()
+        return x
+
+    def get_max(self):
+        return self.m.peek()
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_stack_usage(self):
+        max_stack = MaxStack()
+
+        max_stack.push(5)
+
+        actual = max_stack.get_max()
+        expected = 5
+        self.assertEqual(actual, expected)
+
+        max_stack.push(4)
+        max_stack.push(7)
+        max_stack.push(7)
+        max_stack.push(8)
+
+        actual = max_stack.get_max()
+        expected = 8
+        self.assertEqual(actual, expected)
+
+        actual = max_stack.pop()
+        expected = 8
+        self.assertEqual(actual, expected)
+
+        actual = max_stack.get_max()
+        expected = 7
+        self.assertEqual(actual, expected)
+
+        actual = max_stack.pop()
+        expected = 7
+        self.assertEqual(actual, expected)
+
+        actual = max_stack.get_max()
+        expected = 7
+        self.assertEqual(actual, expected)
+
+        actual = max_stack.pop()
+        expected = 7
+        self.assertEqual(actual, expected)
+
+        actual = max_stack.get_max()
+        expected = 5
+        self.assertEqual(actual, expected)
+
+        actual = max_stack.pop()
+        expected = 4
+        self.assertEqual(actual, expected)
+
+        actual = max_stack.get_max()
+        expected = 5
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/linked-list-cycles.py b/users/wpcarro/scratch/data_structures_and_algorithms/linked-list-cycles.py
new file mode 100644
index 0000000000..75a4b99394
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/linked-list-cycles.py
@@ -0,0 +1,88 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+def contains_cycle(x):
+    if not x:
+        return False
+    elif not x.next:
+        return False
+
+    a = x
+    b = x.next
+
+    while b.next:
+        if a == b:
+            return True
+
+        a = a.next
+        b = b.next.next
+
+    return False
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    class LinkedListNode(object):
+        def __init__(self, value, next=None):
+            self.value = value
+            self.next = next
+
+    def test_linked_list_with_no_cycle(self):
+        fourth = Test.LinkedListNode(4)
+        third = Test.LinkedListNode(3, fourth)
+        second = Test.LinkedListNode(2, third)
+        first = Test.LinkedListNode(1, second)
+        result = contains_cycle(first)
+        self.assertFalse(result)
+
+    def test_cycle_loops_to_beginning(self):
+        fourth = Test.LinkedListNode(4)
+        third = Test.LinkedListNode(3, fourth)
+        second = Test.LinkedListNode(2, third)
+        first = Test.LinkedListNode(1, second)
+        fourth.next = first
+        result = contains_cycle(first)
+        self.assertTrue(result)
+
+    def test_cycle_loops_to_middle(self):
+        fifth = Test.LinkedListNode(5)
+        fourth = Test.LinkedListNode(4, fifth)
+        third = Test.LinkedListNode(3, fourth)
+        second = Test.LinkedListNode(2, third)
+        first = Test.LinkedListNode(1, second)
+        fifth.next = third
+        result = contains_cycle(first)
+        self.assertTrue(result)
+
+    def test_two_node_cycle_at_end(self):
+        fifth = Test.LinkedListNode(5)
+        fourth = Test.LinkedListNode(4, fifth)
+        third = Test.LinkedListNode(3, fourth)
+        second = Test.LinkedListNode(2, third)
+        first = Test.LinkedListNode(1, second)
+        fifth.next = fourth
+        result = contains_cycle(first)
+        self.assertTrue(result)
+
+    def test_empty_list(self):
+        result = contains_cycle(None)
+        self.assertFalse(result)
+
+    def test_one_element_linked_list_no_cycle(self):
+        first = Test.LinkedListNode(1)
+        result = contains_cycle(first)
+        self.assertFalse(result)
+
+    def test_one_element_linked_list_cycle(self):
+        first = Test.LinkedListNode(1)
+        first.next = first
+        result = contains_cycle(first)
+        self.assertTrue(result)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/memo.py b/users/wpcarro/scratch/data_structures_and_algorithms/memo.py
new file mode 100644
index 0000000000..44ea93e1bd
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/memo.py
@@ -0,0 +1,60 @@
+import time
+import random
+from heapq import heappush, heappop
+
+
+class Memo(object):
+    def __init__(self, size=1):
+        """
+        Create a key-value data-structure that will never exceed `size`
+        members. Memo evicts the least-recently-accessed elements from itself
+        before adding inserting new key-value pairs.
+        """
+        if size <= 0:
+            raise Exception("We do not support an empty memo")
+        self.xs = {}
+        self.heap = [(0, None)] * size
+
+    def contains(self, k):
+        """
+        Return true if key `k` exists in the Memo.
+        """
+        return k in self.xs
+
+    def get(self, k):
+        """
+        Return the memoized item at key `k`.
+        """
+        # "touch" the element in the heap
+        return self.xs[k]
+
+    def set(self, k, v):
+        """
+        Memoize value `v` at key `k`.
+        """
+        _, to_evict = heappop(self.heap)
+        if to_evict != None:
+            del self.xs[to_evict]
+        heappush(self.heap, (time.time(), k))
+        self.xs[k] = v
+
+
+memo = Memo(size=10)
+
+
+def f(x):
+    """
+    Compute some mysterious, expensive function.
+    """
+    if memo.contains(x):
+        print("Hit.\t\tf({})".format(x))
+        return memo.get(x)
+    else:
+        print("Computing...\tf({})".format(x))
+        time.sleep(0.25)
+        res = random.randint(0, 10)
+        memo.set(x, res)
+        return res
+
+
+[f(random.randint(0, 10)) for _ in range(10)]
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/merge-sort.py b/users/wpcarro/scratch/data_structures_and_algorithms/merge-sort.py
new file mode 100644
index 0000000000..6dbe0fa0f3
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/merge-sort.py
@@ -0,0 +1,28 @@
+
+
+
+# merge :: [a] -> [a] -> [a]
+# merge([], []): []
+# merge(xs, []): xs
+# merge([], ys): ys
+# merge(xs@[x|xs'], ys@[y|ys'])
+#   when y =< x: cons(y, merge(xs, ys'))
+#   when x < y:  cons(x, merge(xs', ys))
+def merge(xs, ys):
+    if xs == [] and ys == []:
+        return []
+    elif ys == []:
+        return xs
+    elif xs == []:
+        return ys
+    else:
+        x = xs[0]
+        y = ys[0]
+
+        if y <= x:
+            return [y] + merge(xs, ys[1:])
+        else:
+            return [x] + merge(xs[1:], ys)
+        
+print(merge([3, 4, 6, 10, 11, 15],
+            [1, 5, 8, 12, 14, 19]))
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/merging-ranges.py b/users/wpcarro/scratch/data_structures_and_algorithms/merging-ranges.py
new file mode 100644
index 0000000000..4e3604d5bc
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/merging-ranges.py
@@ -0,0 +1,94 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+# do_merge_ranges :: [(Int, Int)] -> [(Int, Int)] -> [(Int, Int)]
+def do_merge_ranges(prev, xs):
+    if len(xs) == 0:
+        return prev
+    elif len(xs) == 1:
+        return prev + xs
+    else:
+        a1, a2 = xs[0]
+        b1, b2 = xs[1]
+        rest = xs[2:]
+        if b1 <= a2:
+            return do_merge_ranges(prev, [(a1, max(a2, b2))] + rest)
+        else:
+            return do_merge_ranges(prev + [(a1, a2)], [(b1, b2)] + rest)
+
+
+# merge_ranges :: [(Int, Int)] -> [(Int, Int)]
+def merge_ranges(xs):
+    xs = xs[:]
+    xs.sort()
+    return do_merge_ranges([], xs)
+
+
+# merge_ranges_b :: [(Int, Int)] -> [(Int, Int)]
+def merge_ranges_b(xs):
+    fi = 0
+    ci = 1
+    result = []
+    xs = xs[:]
+    xs.sort()
+    while ci < len(xs):
+        while ci < len(xs) and xs[ci][0] <= xs[fi][1]:
+            xs[fi] = xs[fi][0], max(xs[ci][1], xs[fi][1])
+            ci += 1
+        result.append(xs[fi])
+        fi = ci
+        ci += 1
+    if fi < len(xs):
+        result.append(xs[fi])
+    return result
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_meetings_overlap(self):
+        actual = merge_ranges([(1, 3), (2, 4)])
+        expected = [(1, 4)]
+        self.assertEqual(actual, expected)
+
+    def test_meetings_touch(self):
+        actual = merge_ranges([(5, 6), (6, 8)])
+        expected = [(5, 8)]
+        self.assertEqual(actual, expected)
+
+    def test_meeting_contains_other_meeting(self):
+        actual = merge_ranges([(1, 8), (2, 5)])
+        expected = [(1, 8)]
+        self.assertEqual(actual, expected)
+
+    def test_meetings_stay_separate(self):
+        actual = merge_ranges([(1, 3), (4, 8)])
+        expected = [(1, 3), (4, 8)]
+        self.assertEqual(actual, expected)
+
+    def test_multiple_merged_meetings(self):
+        actual = merge_ranges([(1, 4), (2, 5), (5, 8)])
+        expected = [(1, 8)]
+        self.assertEqual(actual, expected)
+
+    def test_meetings_not_sorted(self):
+        actual = merge_ranges([(5, 8), (1, 4), (6, 8)])
+        expected = [(1, 4), (5, 8)]
+        self.assertEqual(actual, expected)
+
+    def test_one_long_meeting_contains_smaller_meetings(self):
+        actual = merge_ranges([(1, 10), (2, 5), (6, 8), (9, 10), (10, 12)])
+        expected = [(1, 12)]
+        self.assertEqual(actual, expected)
+
+    def test_sample_input(self):
+        actual = merge_ranges([(0, 1), (3, 5), (4, 8), (10, 12), (9, 10)])
+        expected = [(0, 1), (3, 8), (9, 12)]
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/mesh-message.gv b/users/wpcarro/scratch/data_structures_and_algorithms/mesh-message.gv
new file mode 100644
index 0000000000..1e67c3954f
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/mesh-message.gv
@@ -0,0 +1,11 @@
+strict graph {
+    Min -- {William, Jayden, Omar}
+    William -- {Min, Noam}
+    Jayden -- {Min, Amelia, Ren, Noam}
+    Adam -- {Amelia, Miguel, Sofia, Lucas}
+    Ren -- {Jayden, Omar}
+    Amelia -- {Jayden, Adam, Miguel}
+    Miguel -- {Amelia, Adam, Liam, Nathan}
+    Noam -- {Nathan, Jayden, William}
+    Omar -- {Ren, Min, Scott}
+}
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/mesh-message.py b/users/wpcarro/scratch/data_structures_and_algorithms/mesh-message.py
new file mode 100644
index 0000000000..c9d7d9d741
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/mesh-message.py
@@ -0,0 +1,97 @@
+import unittest
+from collections import deque
+
+
+################################################################################
+# Solution
+################################################################################
+# get_path :: G(V, E) -> V -> V -> Maybe([V])
+def get_path(g, src, dst):
+    q = deque()
+    result = None
+    seen = set()
+    q.append(([], src))
+
+    if src not in g or dst not in g:
+        raise Exception
+
+    while q:
+        p, node = q.popleft()
+
+        seen.add(node)
+
+        if node == dst:
+            if not result:
+                result = p + [node]
+            elif len(p + [node]) < len(result):
+                result = p + [node]
+        else:
+            if node not in g:
+                raise Exception
+            for x in g.get(node):
+                if not x in seen:
+                    q.append((p + [node], x))
+
+    return result
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def setUp(self):
+        self.graph = {
+            'a': ['b', 'c', 'd'],
+            'b': ['a', 'd'],
+            'c': ['a', 'e'],
+            'd': ['a', 'b'],
+            'e': ['c'],
+            'f': ['g'],
+            'g': ['f'],
+        }
+
+    def test_two_hop_path_1(self):
+        actual = get_path(self.graph, 'a', 'e')
+        expected = ['a', 'c', 'e']
+        self.assertEqual(actual, expected)
+
+    def test_two_hop_path_2(self):
+        actual = get_path(self.graph, 'd', 'c')
+        expected = ['d', 'a', 'c']
+        self.assertEqual(actual, expected)
+
+    def test_one_hop_path_1(self):
+        actual = get_path(self.graph, 'a', 'c')
+        expected = ['a', 'c']
+        self.assertEqual(actual, expected)
+
+    def test_one_hop_path_2(self):
+        actual = get_path(self.graph, 'f', 'g')
+        expected = ['f', 'g']
+        self.assertEqual(actual, expected)
+
+    def test_one_hop_path_3(self):
+        actual = get_path(self.graph, 'g', 'f')
+        expected = ['g', 'f']
+        self.assertEqual(actual, expected)
+
+    def test_zero_hop_path(self):
+        actual = get_path(self.graph, 'a', 'a')
+        expected = ['a']
+        self.assertEqual(actual, expected)
+
+    def test_no_path(self):
+        actual = get_path(self.graph, 'a', 'f')
+        expected = None
+        self.assertEqual(actual, expected)
+
+    def test_start_node_not_present(self):
+        with self.assertRaises(Exception):
+            get_path(self.graph, 'h', 'a')
+
+    def test_end_node_not_present(self):
+        with self.assertRaises(Exception):
+            get_path(self.graph, 'a', 'h')
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/norman.py b/users/wpcarro/scratch/data_structures_and_algorithms/norman.py
new file mode 100644
index 0000000000..379ba92abb
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/norman.py
@@ -0,0 +1,78 @@
+
+
+
+# Write a function with the following type signature:L
+# equal? :: String -> String -> Bool
+#
+# Determine equality between two inputs with backspace characters encoded as
+# "<".
+
+################################################################################
+# Solution 1
+################################################################################
+
+# from collections import deque
+
+# def equal(a, b):
+#     sa = deque()
+#     sb = deque()
+
+#     for c in a:
+#         if c == '<':
+#             sa.pop()
+#         else:
+#             sa.append(c)
+
+#     for c in b:
+#         if c == '<':
+#             sb.pop()
+#         else:
+#             sb.append(c)
+
+#     return sa == sb
+
+################################################################################
+# Solution 2
+################################################################################
+
+def handle_dels(num_dels, i, xs):
+    if i < 0:
+        return -1
+
+    while xs[i] == '<':
+        num_dels += 1
+        i -= 1
+
+    while num_dels > 0 and xs[i] != '<':
+        num_dels -= 1
+        i -= 1
+
+    if xs[i] == '<':
+        return handle_dels(num_dels, i, xs)
+    else:
+        return i
+
+def update_index(i, xs):
+    # TODO: Indexing into non-available parts of a string.
+    if xs[i] != '<' and xs[i - 1] != '<':
+        return i - 1
+
+    elif xs[i - 1] == '<':
+        return handle_dels(0, i - 1, xs)
+
+def equal(a, b):
+    ia = len(a) - 1
+    ib = len(b) - 1
+
+    while ia >= 0 and ib >= 0:
+        if a[ia] != b[ib]:
+            return False
+        ia = update_index(ia, a)
+        ib = update_index(ib, b)
+
+    if ia != 0:
+        return update_index(ia, a) <= -1
+    if ib != 0:
+        return update_index(ib, b) <= -1
+
+    return True
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/nth-fibonacci.py b/users/wpcarro/scratch/data_structures_and_algorithms/nth-fibonacci.py
new file mode 100644
index 0000000000..cdb2846ea3
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/nth-fibonacci.py
@@ -0,0 +1,59 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+def fib(n):
+    """This should be accomplishable in O(1) space."""
+    if n in {0, 1}:
+        return n
+    a = 0  # i = 0
+    b = 1  # i = 1
+    for x in range(2, n + 1):
+        result = a + b
+        a = b
+        b = result
+    return result
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_zeroth_fibonacci(self):
+        actual = fib(0)
+        expected = 0
+        self.assertEqual(actual, expected)
+
+    def test_first_fibonacci(self):
+        actual = fib(1)
+        expected = 1
+        self.assertEqual(actual, expected)
+
+    def test_second_fibonacci(self):
+        actual = fib(2)
+        expected = 1
+        self.assertEqual(actual, expected)
+
+    def test_third_fibonacci(self):
+        actual = fib(3)
+        expected = 2
+        self.assertEqual(actual, expected)
+
+    def test_fifth_fibonacci(self):
+        actual = fib(5)
+        expected = 5
+        self.assertEqual(actual, expected)
+
+    def test_tenth_fibonacci(self):
+        actual = fib(10)
+        expected = 55
+        self.assertEqual(actual, expected)
+
+    def test_negative_fibonacci(self):
+        with self.assertRaises(Exception):
+            fib(-1)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/optimal-stopping.py b/users/wpcarro/scratch/data_structures_and_algorithms/optimal-stopping.py
new file mode 100644
index 0000000000..af13239941
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/optimal-stopping.py
@@ -0,0 +1,49 @@
+from random import choice
+from math import floor
+
+# Applying Chapter 1 from "Algorithms to Live By", which describes optimal
+# stopping problems. Technically this simulation is invalid because the
+# `candidates` function takes a lower bound and an upper bound, which allows us
+# to know the cardinal number of an individual candidates. The "look then leap"
+# algorithm is ideal for no-information games - i.e. games when upper and lower
+# bounds aren't known. The `look_then_leap/1` function is ignorant of this
+# information, so it behaves as if in a no-information game. Strangely enough,
+# this algorithm will pick the best candidate 37% of the time.
+#
+# Chapter 1 describes two algorithms:
+# 1. Look-then-leap: ordinal numbers - i.e. no-information games. Look-then-leap
+#    finds the best candidate 37% of the time.
+# 2. Threshold: cardinal numbers - i.e. where upper and lower bounds are
+#    known. The Threshold algorithm finds the best candidate ~55% of the time.
+#
+# All of this and more can be studied as "optimal stopping theory". This applies
+# to finding a spouse, parking a car, picking an apartment in a city, and more.
+
+
+# candidates :: Int -> Int -> Int -> [Int]
+def candidates(lb, ub, ct):
+    xs = list(range(lb, ub + 1))
+    return [choice(xs) for _ in range(ct)]
+
+
+# look_then_leap :: [Integer] -> Integer
+def look_then_leap(candidates):
+    best = candidates[0]
+    seen_ct = 1
+    ignore_ct = floor(len(candidates) * 0.37)
+    for x in candidates[1:]:
+        if ignore_ct > 0:
+            ignore_ct -= 1
+            best = max(best, x)
+        else:
+            if x > best:
+                print('Choosing the {} candidate.'.format(seen_ct))
+                return x
+        seen_ct += 1
+    print('You may have waited too long.')
+    return candidates[-1]
+
+
+candidates = candidates(1, 100, 100)
+print(candidates)
+print(look_then_leap(candidates))
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/perm-tree.py b/users/wpcarro/scratch/data_structures_and_algorithms/perm-tree.py
new file mode 100644
index 0000000000..0eb389c26b
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/perm-tree.py
@@ -0,0 +1,83 @@
+import unittest
+
+
+################################################################################
+# Answer
+################################################################################
+class Node(object):
+    def __init__(self, value, children=set()):
+        self.value = value
+        self.children = children
+
+
+# treeify :: Char -> Set(Char) -> Node(Char)
+def treeify(x, xs):
+    return Node(x, [treeify(c, xs - {c}) for c in xs])
+
+
+# dft :: Node(Char) -> [String]
+def dft(node):
+    result = []
+    s = []
+
+    s.append(('', node))
+
+    while s:
+        p, n = s.pop()
+        p += str(n.value)
+
+        if not n.children:
+            result.append(p)
+        else:
+            for c in n.children:
+                s.append((p, c))
+
+    return result
+
+
+# main :: String -> Set(String)
+def get_permutations(xs):
+    if xs == '':
+        return set([''])
+
+    ys = set(xs)
+    trees = []
+
+    for y in ys:
+        trees.append(treeify(y, ys - {y}))
+
+    result = set()
+
+    for t in trees:
+        for d in dft(t):
+            result.add(d)
+
+    return result
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_empty_string(self):
+        actual = get_permutations('')
+        expected = set([''])
+        self.assertEqual(actual, expected)
+
+    def test_one_character_string(self):
+        actual = get_permutations('a')
+        expected = set(['a'])
+        self.assertEqual(actual, expected)
+
+    def test_two_character_string(self):
+        actual = get_permutations('ab')
+        expected = set(['ab', 'ba'])
+        self.assertEqual(actual, expected)
+
+    def test_three_character_string(self):
+        actual = get_permutations('abc')
+        expected = set(['abc', 'acb', 'bac', 'bca', 'cab', 'cba'])
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/permutation-palindrome.py b/users/wpcarro/scratch/data_structures_and_algorithms/permutation-palindrome.py
new file mode 100644
index 0000000000..0a2136a408
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/permutation-palindrome.py
@@ -0,0 +1,49 @@
+from collections import Counter
+import unittest
+
+
+################################################################################
+# Impl
+################################################################################
+# palindromifiable :: String -> Boolean
+def has_palindrome_permutation(x):
+    bag = Counter(x)
+    odd_entries_ct = 0
+
+    for _, y in bag.items():
+        if y % 2 != 0:
+            odd_entries_ct += 1
+
+    return odd_entries_ct in {0, 1}
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_permutation_with_odd_number_of_chars(self):
+        result = has_palindrome_permutation('aabcbcd')
+        self.assertTrue(result)
+
+    def test_permutation_with_even_number_of_chars(self):
+        result = has_palindrome_permutation('aabccbdd')
+        self.assertTrue(result)
+
+    def test_no_permutation_with_odd_number_of_chars(self):
+        result = has_palindrome_permutation('aabcd')
+        self.assertFalse(result)
+
+    def test_no_permutation_with_even_number_of_chars(self):
+        result = has_palindrome_permutation('aabbcd')
+        self.assertFalse(result)
+
+    def test_empty_string(self):
+        result = has_palindrome_permutation('')
+        self.assertTrue(result)
+
+    def test_one_character_string(self):
+        result = has_palindrome_permutation('a')
+        self.assertTrue(result)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/permutations.py b/users/wpcarro/scratch/data_structures_and_algorithms/permutations.py
new file mode 100644
index 0000000000..fc2c1ef7ee
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/permutations.py
@@ -0,0 +1,55 @@
+class Node(object):
+    # ctor :: a -> [a] -> Node(a)
+    def __init__(self, value, children=[]):
+        self.value = value
+        self.children = children
+
+
+# is_leaf :: Node(a) -> Boolean
+def is_leaf(node):
+    return len(node.children) == 0
+
+
+# enumerate :: Node(a) -> Set(List(a))
+def enumerate(node):
+    current = []
+    result = []
+    q = []
+
+    q.append(node)
+
+    while q:
+        x = q.pop()
+        print(x.value)
+
+        for c in x.children:
+            q.append(c)
+
+        current.append(x.value)
+        print(current)
+
+        if is_leaf(x):
+            result.append(current)
+            print("Reseting current")
+            current = []
+
+    return result
+
+
+node = Node('root', [
+    Node('a', [
+        Node('b', [Node('c')]),
+        Node('c', [Node('b')]),
+    ]),
+    Node('b', [
+        Node('a', [Node('c')]),
+        Node('c', [Node('a')]),
+    ]),
+    Node('c', [
+        Node('a', [Node('b')]),
+        Node('b', [Node('a')]),
+    ])
+])
+
+print('----------')
+print(enumerate(node))
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/plot.py b/users/wpcarro/scratch/data_structures_and_algorithms/plot.py
new file mode 100644
index 0000000000..5601891a0d
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/plot.py
@@ -0,0 +1,9 @@
+import numpy as np
+import matplotlib.pyplot as plt
+
+rng = np.random.RandomState(10)  # deterministic random data
+a = np.hstack((rng.normal(size=1000), rng.normal(loc=5, scale=2, size=1000)))
+_ = plt.hist(a, bins='auto')  # arguments are passed to np.histogram
+plt.title("Histogram with 'auto' bins")
+Text(0.5, 1.0, "Histogram with 'auto' bins")
+plt.show()
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/product-of-other-numbers.py b/users/wpcarro/scratch/data_structures_and_algorithms/product-of-other-numbers.py
new file mode 100644
index 0000000000..d05e82d42b
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/product-of-other-numbers.py
@@ -0,0 +1,68 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+# f :: [Int] -> [Int]
+def get_products_of_all_ints_except_at_index(xs):
+    if len(xs) in {0, 1}:
+        raise Exception
+
+    ct = len(xs)
+    lefts = [1] * ct
+    rights = [1] * ct
+    result = []
+
+    for i in range(1, ct):
+        lefts[i] = lefts[i - 1] * xs[i - 1]
+    for i in range(ct - 2, -1, -1):
+        rights[i] = rights[i + 1] * xs[i + 1]
+
+    return [lefts[i] * rights[i] for i in range(ct)]
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_small_list(self):
+        actual = get_products_of_all_ints_except_at_index([1, 2, 3])
+        expected = [6, 3, 2]
+        self.assertEqual(actual, expected)
+
+    def test_longer_list(self):
+        actual = get_products_of_all_ints_except_at_index([8, 2, 4, 3, 1, 5])
+        expected = [120, 480, 240, 320, 960, 192]
+        self.assertEqual(actual, expected)
+
+    def test_list_has_one_zero(self):
+        actual = get_products_of_all_ints_except_at_index([6, 2, 0, 3])
+        expected = [0, 0, 36, 0]
+        self.assertEqual(actual, expected)
+
+    def test_list_has_two_zeros(self):
+        actual = get_products_of_all_ints_except_at_index([4, 0, 9, 1, 0])
+        expected = [0, 0, 0, 0, 0]
+        self.assertEqual(actual, expected)
+
+    def test_one_negative_number(self):
+        actual = get_products_of_all_ints_except_at_index([-3, 8, 4])
+        expected = [32, -12, -24]
+        self.assertEqual(actual, expected)
+
+    def test_all_negative_numbers(self):
+        actual = get_products_of_all_ints_except_at_index([-7, -1, -4, -2])
+        expected = [-8, -56, -14, -28]
+        self.assertEqual(actual, expected)
+
+    def test_error_with_empty_list(self):
+        with self.assertRaises(Exception):
+            get_products_of_all_ints_except_at_index([])
+
+    def test_error_with_one_number(self):
+        with self.assertRaises(Exception):
+            get_products_of_all_ints_except_at_index([1])
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/queue-two-stacks.py b/users/wpcarro/scratch/data_structures_and_algorithms/queue-two-stacks.py
new file mode 100644
index 0000000000..63da08ebf7
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/queue-two-stacks.py
@@ -0,0 +1,66 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+class QueueTwoStacks(object):
+    def __init__(self):
+        self.a = []
+        self.b = []
+
+    def enqueue(self, x):
+        self.a.append(x)
+
+    def dequeue(self):
+        if self.b:
+            return self.b.pop()
+        else:
+            while self.a:
+                self.b.append(self.a.pop())
+            return self.dequeue()
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_basic_queue_operations(self):
+        queue = QueueTwoStacks()
+        queue.enqueue(1)
+        queue.enqueue(2)
+        queue.enqueue(3)
+        actual = queue.dequeue()
+        expected = 1
+        self.assertEqual(actual, expected)
+        actual = queue.dequeue()
+        expected = 2
+        self.assertEqual(actual, expected)
+        queue.enqueue(4)
+        actual = queue.dequeue()
+        expected = 3
+        self.assertEqual(actual, expected)
+        actual = queue.dequeue()
+        expected = 4
+        self.assertEqual(actual, expected)
+
+    def test_error_when_dequeue_from_new_queue(self):
+        queue = QueueTwoStacks()
+        with self.assertRaises(Exception):
+            queue.dequeue()
+
+    def test_error_when_dequeue_from_empty_queue(self):
+        queue = QueueTwoStacks()
+        queue.enqueue(1)
+        queue.enqueue(2)
+        actual = queue.dequeue()
+        expected = 1
+        self.assertEqual(actual, expected)
+        actual = queue.dequeue()
+        expected = 2
+        self.assertEqual(actual, expected)
+        with self.assertRaises(Exception):
+            queue.dequeue()
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/rectangular-love.py b/users/wpcarro/scratch/data_structures_and_algorithms/rectangular-love.py
new file mode 100644
index 0000000000..47c0f53979
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/rectangular-love.py
@@ -0,0 +1,246 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+# bottom :: Rectangle -> Int
+def bottom(x):
+    return x.get('bottom_y')
+
+
+# top :: Rectangle -> Int
+def top(x):
+    return bottom(x) + x.get('height')
+
+
+# left :: Rectangle -> Int
+def left(x):
+    return x.get('left_x')
+
+
+# right :: Rectangle -> Int
+def right(x):
+    return left(x) + x.get('width')
+
+
+# sort_highest :: Rectangle -> Rectangle -> (Rectangle, Rectangle)
+def sort_highest(x, y):
+    if top(x) >= top(y):
+        return x, y
+    else:
+        return y, x
+
+
+# sort_leftmost :: Rectangle -> Rectangle -> (Rectangle, Rectangle)
+def sort_leftmost(x, y):
+    if left(x) <= left(y):
+        return x, y
+    else:
+        return y, x
+
+
+# rectify :: Int -> Int -> Int -> Int -> Rectify
+def rectify(top=None, bottom=None, left=None, right=None):
+    assert top >= bottom
+    assert left <= right
+    return {
+        'left_x': left,
+        'bottom_y': bottom,
+        'width': right - left,
+        'height': top - bottom,
+    }
+
+
+# empty_rect :: Rectangle
+def empty_rect():
+    return {
+        'left_x': None,
+        'bottom_y': None,
+        'width': None,
+        'height': None,
+    }
+
+
+# find_rectangular_overlap :: Rectangle -> Rectangle -> Maybe(Rectangle)
+def find_rectangular_overlap(x, y):
+    ha, hb = sort_highest(x, y)
+    la, lb = sort_leftmost(x, y)
+
+    if bottom(hb) <= top(hb) <= bottom(ha) <= top(ha):
+        return empty_rect()
+
+    if left(la) <= right(la) <= left(lb) <= right(lb):
+        return empty_rect()
+
+    # We should have an intersection here.
+    verts = [bottom(ha), top(ha), bottom(hb), top(hb)]
+    verts.sort()
+    horzs = [left(la), right(la), left(lb), right(lb)]
+    horzs.sort()
+    return rectify(top=verts[2],
+                   bottom=verts[1],
+                   left=horzs[1],
+                   right=horzs[2])
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_overlap_along_both_axes(self):
+        rect1 = {
+            'left_x': 1,
+            'bottom_y': 1,
+            'width': 6,
+            'height': 3,
+        }
+        rect2 = {
+            'left_x': 5,
+            'bottom_y': 2,
+            'width': 3,
+            'height': 6,
+        }
+        expected = {
+            'left_x': 5,
+            'bottom_y': 2,
+            'width': 2,
+            'height': 2,
+        }
+        actual = find_rectangular_overlap(rect1, rect2)
+        self.assertEqual(actual, expected)
+
+    def test_one_rectangle_inside_another(self):
+        rect1 = {
+            'left_x': 1,
+            'bottom_y': 1,
+            'width': 6,
+            'height': 6,
+        }
+        rect2 = {
+            'left_x': 3,
+            'bottom_y': 3,
+            'width': 2,
+            'height': 2,
+        }
+        expected = {
+            'left_x': 3,
+            'bottom_y': 3,
+            'width': 2,
+            'height': 2,
+        }
+        actual = find_rectangular_overlap(rect1, rect2)
+        self.assertEqual(actual, expected)
+
+    def test_both_rectangles_the_same(self):
+        rect1 = {
+            'left_x': 2,
+            'bottom_y': 2,
+            'width': 4,
+            'height': 4,
+        }
+        rect2 = {
+            'left_x': 2,
+            'bottom_y': 2,
+            'width': 4,
+            'height': 4,
+        }
+        expected = {
+            'left_x': 2,
+            'bottom_y': 2,
+            'width': 4,
+            'height': 4,
+        }
+        actual = find_rectangular_overlap(rect1, rect2)
+        self.assertEqual(actual, expected)
+
+    def test_touch_on_horizontal_edge(self):
+        rect1 = {
+            'left_x': 1,
+            'bottom_y': 2,
+            'width': 3,
+            'height': 4,
+        }
+        rect2 = {
+            'left_x': 2,
+            'bottom_y': 6,
+            'width': 2,
+            'height': 2,
+        }
+        expected = {
+            'left_x': None,
+            'bottom_y': None,
+            'width': None,
+            'height': None,
+        }
+        actual = find_rectangular_overlap(rect1, rect2)
+        self.assertEqual(actual, expected)
+
+    def test_touch_on_vertical_edge(self):
+        rect1 = {
+            'left_x': 1,
+            'bottom_y': 2,
+            'width': 3,
+            'height': 4,
+        }
+        rect2 = {
+            'left_x': 4,
+            'bottom_y': 3,
+            'width': 2,
+            'height': 2,
+        }
+        expected = {
+            'left_x': None,
+            'bottom_y': None,
+            'width': None,
+            'height': None,
+        }
+        actual = find_rectangular_overlap(rect1, rect2)
+        self.assertEqual(actual, expected)
+
+    def test_touch_at_a_corner(self):
+        rect1 = {
+            'left_x': 1,
+            'bottom_y': 1,
+            'width': 2,
+            'height': 2,
+        }
+        rect2 = {
+            'left_x': 3,
+            'bottom_y': 3,
+            'width': 2,
+            'height': 2,
+        }
+        expected = {
+            'left_x': None,
+            'bottom_y': None,
+            'width': None,
+            'height': None,
+        }
+        actual = find_rectangular_overlap(rect1, rect2)
+        self.assertEqual(actual, expected)
+
+    def test_no_overlap(self):
+        rect1 = {
+            'left_x': 1,
+            'bottom_y': 1,
+            'width': 2,
+            'height': 2,
+        }
+        rect2 = {
+            'left_x': 4,
+            'bottom_y': 6,
+            'width': 3,
+            'height': 6,
+        }
+        expected = {
+            'left_x': None,
+            'bottom_y': None,
+            'width': None,
+            'height': None,
+        }
+        actual = find_rectangular_overlap(rect1, rect2)
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/recursive-string-permutations.py b/users/wpcarro/scratch/data_structures_and_algorithms/recursive-string-permutations.py
new file mode 100644
index 0000000000..70461ddf5d
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/recursive-string-permutations.py
@@ -0,0 +1,37 @@
+import unittest
+
+
+################################################################################
+# Implementation
+################################################################################
+# get_permutations :: String -> Set(String)
+def get_permutations(string):
+    pass
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_empty_string(self):
+        actual = get_permutations('')
+        expected = set([''])
+        self.assertEqual(actual, expected)
+
+    def test_one_character_string(self):
+        actual = get_permutations('a')
+        expected = set(['a'])
+        self.assertEqual(actual, expected)
+
+    def test_two_character_string(self):
+        actual = get_permutations('ab')
+        expected = set(['ab', 'ba'])
+        self.assertEqual(actual, expected)
+
+    def test_three_character_string(self):
+        actual = get_permutations('abc')
+        expected = set(['abc', 'acb', 'bac', 'bca', 'cab', 'cba'])
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/reverse-linked-list.py b/users/wpcarro/scratch/data_structures_and_algorithms/reverse-linked-list.py
new file mode 100644
index 0000000000..b7396b20ce
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/reverse-linked-list.py
@@ -0,0 +1,79 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+# reverse :: List(a) -> List(a)
+def reverse(node):
+    curr = node
+    prev = None
+    while curr:
+        nxt = curr.next
+        curr.next = prev
+        prev = curr
+        curr = nxt
+    # Make sure to understand the spec! Debugging takes time. Rewriting takes
+    # time.
+    return prev
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    class LinkedListNode(object):
+        def __init__(self, value, next=None):
+            self.value = value
+            self.next = next
+
+        def get_values(self):
+            node = self
+            values = []
+            while node is not None:
+                values.append(node.value)
+                node = node.next
+            return values
+
+    def test_short_linked_list(self):
+        second = Test.LinkedListNode(2)
+        first = Test.LinkedListNode(1, second)
+
+        result = reverse(first)
+        self.assertIsNotNone(result)
+
+        actual = result.get_values()
+        expected = [2, 1]
+        self.assertEqual(actual, expected)
+
+    def test_long_linked_list(self):
+        sixth = Test.LinkedListNode(6)
+        fifth = Test.LinkedListNode(5, sixth)
+        fourth = Test.LinkedListNode(4, fifth)
+        third = Test.LinkedListNode(3, fourth)
+        second = Test.LinkedListNode(2, third)
+        first = Test.LinkedListNode(1, second)
+
+        result = reverse(first)
+        self.assertIsNotNone(result)
+
+        actual = result.get_values()
+        expected = [6, 5, 4, 3, 2, 1]
+        self.assertEqual(actual, expected)
+
+    def test_one_element_linked_list(self):
+        first = Test.LinkedListNode(1)
+
+        result = reverse(first)
+        self.assertIsNotNone(result)
+
+        actual = result.get_values()
+        expected = [1]
+        self.assertEqual(actual, expected)
+
+    def test_empty_linked_list(self):
+        result = reverse(None)
+        self.assertIsNone(result)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/reverse-words.py b/users/wpcarro/scratch/data_structures_and_algorithms/reverse-words.py
new file mode 100644
index 0000000000..5df12ebabd
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/reverse-words.py
@@ -0,0 +1,181 @@
+from collections import deque
+import unittest
+
+################################################################################
+# Solution
+################################################################################
+
+
+def rev(xs, i, j):
+    """Reverse xs in place from [i, j]"""
+    while i < j:
+        xs[i], xs[j] = xs[j], xs[i]
+        i += 1
+        j -= 1
+
+
+def rotate(xs, n, i=None, j=None):
+    """Mutably rotates list, xs, n times. Positive n values rotate right while
+    negative n values rotate left. Rotate within window [i, j]."""
+    i = i or 0
+    j = j or len(xs) - 1
+    ct = j - i
+
+    if n < 0:
+        n = abs(n)
+        p = i + n - 1
+        rev(xs, i, p)
+        rev(xs, p + 1, j)
+        rev(xs, i, j)
+    else:
+        p = j - (n - 1)
+        rev(xs, p, j)
+        rev(xs, i, p - 1)
+        rev(xs, i, j)
+    return xs
+
+
+def rev_words(xs, i, j):
+    if j + 1 == len(xs):
+        return 0
+
+    while j + 1 < len(xs):
+        while j + 1 < len(xs) and xs[j + 1] != ' ':
+            j += 1
+
+        rev(xs, i, j)
+        j += 2
+        i = j
+
+    return 0
+
+
+def reverse_words(xs):
+    # first reverse everything
+    rev(xs, 0, len(xs) - 1)
+    return rev_words(xs, 0, 0)
+
+
+def reverse_words_bak(xs, i=None, j=None):
+    i = i or 0
+    j = j or len(xs) - 1
+    w0, w1 = [], []
+
+    if i >= j:
+        return 0
+
+    pi = i
+    while pi < len(xs) and xs[pi] != ' ':
+        w0.append(xs[pi])
+        pi += 1
+
+    if pi == len(xs):
+        return 0
+
+    pj = j
+    while xs[pj] != ' ':
+        w1.append(xs[pj])
+        pj -= 1
+
+    d = len(w0) - len(w1)
+
+    rotate(xs, -1 * d, i, j)
+
+    for k in range(len(w1)):
+        xs[i + k] = w1[len(w1) - 1 - k]
+
+    for k in range(len(w0)):
+        xs[j - k] = w0[len(w0) - 1 - k]
+
+    while i != j and xs[i] != ' ' and xs[j] != ' ':
+        i += 1
+        j -= 1
+
+    if i == j:
+        return 0
+
+    elif xs[i] == ' ':
+        while j > 0 and xs[j] != ' ':
+            j -= 1
+        if j == 0:
+            return 0
+    elif xs[j] == ' ':
+        while i < len(xs) and xs[i] != ' ':
+            i += 1
+        if i == len(xs):
+            return 0
+    return reverse_words(xs, i + 1, j - 1)
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_rev(self):
+        xs = [1, 2, 3, 4, 5]
+        rev(xs, 0, len(xs) - 1)
+        self.assertEqual(xs, [5, 4, 3, 2, 1])
+
+    def test_rotate(self):
+        ys = [1, 2, 3, 4, 5]
+        xs = ys[:]
+        self.assertEqual(rotate(xs, 1, 1, 3), [1, 4, 2, 3, 5])
+        xs = ys[:]
+        self.assertEqual(rotate(xs, -1, 1, 3), [1, 3, 4, 2, 5])
+        xs = ys[:]
+        self.assertEqual(rotate(xs, 1), [5, 1, 2, 3, 4])
+        xs = ys[:]
+        self.assertEqual(rotate(xs, -1), [2, 3, 4, 5, 1])
+        xs = ys[:]
+        self.assertEqual(rotate(xs, -2), [3, 4, 5, 1, 2])
+        xs = ys[:]
+        self.assertEqual(rotate(xs, -5), [1, 2, 3, 4, 5])
+        xs = ys[:]
+        self.assertEqual(rotate(xs, 5), [1, 2, 3, 4, 5])
+        xs = ys[:]
+        self.assertEqual(rotate(xs, 3), [3, 4, 5, 1, 2])
+
+    def test_one_word(self):
+        message = list('vault')
+        reverse_words(message)
+        expected = list('vault')
+        self.assertEqual(message, expected)
+
+    def test_two_words(self):
+        message = list('thief cake')
+        reverse_words(message)
+        expected = list('cake thief')
+        self.assertEqual(message, expected)
+
+    def test_three_words(self):
+        message = list('one another get')
+        reverse_words(message)
+        expected = list('get another one')
+        self.assertEqual(message, expected)
+
+    def test_multiple_words_same_length(self):
+        message = list('rat the ate cat the')
+        reverse_words(message)
+        expected = list('the cat ate the rat')
+        self.assertEqual(message, expected)
+
+    def test_multiple_words_different_lengths(self):
+        message = list('at rat house')
+        reverse_words(message)
+        expected = list('house rat at')
+        self.assertEqual(message, expected)
+
+    def test_multiple_words_different_lengths(self):
+        message = list('yummy is cake bundt chocolate')
+        reverse_words(message)
+        expected = list('chocolate bundt cake is yummy')
+        self.assertEqual(message, expected)
+
+    def test_empty_string(self):
+        message = list('')
+        reverse_words(message)
+        expected = list('')
+        self.assertEqual(message, expected)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/second-largest-item-bst.py b/users/wpcarro/scratch/data_structures_and_algorithms/second-largest-item-bst.py
new file mode 100644
index 0000000000..bc167d975a
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/second-largest-item-bst.py
@@ -0,0 +1,179 @@
+import unittest
+from collections import deque
+
+
+################################################################################
+# Implementation
+################################################################################
+def is_leaf(node):
+    return node.left is None and node.right is None
+
+
+def find_largest(node):
+    current = node
+    while current.right is not None:
+        current = current.right
+    return current.value
+
+
+def find_second_largest(node):
+    history = deque()
+    current = node
+
+    while current.right:
+        history.append(current)
+        current = current.right
+
+    if current.left:
+        return find_largest(current.left)
+    elif history:
+        return history.pop().value
+    else:
+        raise TypeError
+
+
+def find_second_largest_backup(node):
+    history = deque()
+    current = node
+
+    # traverse -> largest
+    while current.right:
+        history.append(current)
+        current = current.right
+
+    if current.left:
+        return find_largest(current.left)
+    elif history:
+        return history.pop().value
+    else:
+        raise ArgumentError
+
+
+# Write a iterative version to avoid consuming memory with the call stack.
+# Commenting out the recursive code for now.
+def find_second_largest_backup(node):
+    if node.left is None and node.right is None:
+        raise ArgumentError
+
+    elif node.right is None and is_leaf(node.left):
+        return node.left.value
+
+    # recursion
+    # elif node.right is None:
+    #     return find_largest(node.left)
+
+    # iterative version
+    elif node.right is None:
+        current = node.left
+        while current.right is not None:
+            current = current.right
+        return current.value
+
+    # recursion
+    # TODO: Remove recursion from here.
+    elif not is_leaf(node.right):
+        return find_second_largest(node.right)
+
+    # could do an else here, but let's be more assertive.
+    elif is_leaf(node.right):
+        return node.value
+
+    else:
+        raise ArgumentError
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    class BinaryTreeNode(object):
+        def __init__(self, value):
+            self.value = value
+            self.left = None
+            self.right = None
+
+        def insert_left(self, value):
+            self.left = Test.BinaryTreeNode(value)
+            return self.left
+
+        def insert_right(self, value):
+            self.right = Test.BinaryTreeNode(value)
+            return self.right
+
+    def test_full_tree(self):
+        tree = Test.BinaryTreeNode(50)
+        left = tree.insert_left(30)
+        right = tree.insert_right(70)
+        left.insert_left(10)
+        left.insert_right(40)
+        right.insert_left(60)
+        right.insert_right(80)
+        actual = find_second_largest(tree)
+        expected = 70
+        self.assertEqual(actual, expected)
+
+    def test_largest_has_a_left_child(self):
+        tree = Test.BinaryTreeNode(50)
+        left = tree.insert_left(30)
+        right = tree.insert_right(70)
+        left.insert_left(10)
+        left.insert_right(40)
+        right.insert_left(60)
+        actual = find_second_largest(tree)
+        expected = 60
+        self.assertEqual(actual, expected)
+
+    def test_largest_has_a_left_subtree(self):
+        tree = Test.BinaryTreeNode(50)
+        left = tree.insert_left(30)
+        right = tree.insert_right(70)
+        left.insert_left(10)
+        left.insert_right(40)
+        right_left = right.insert_left(60)
+        right_left_left = right_left.insert_left(55)
+        right_left.insert_right(65)
+        right_left_left.insert_right(58)
+        actual = find_second_largest(tree)
+        expected = 65
+        self.assertEqual(actual, expected)
+
+    def test_second_largest_is_root_node(self):
+        tree = Test.BinaryTreeNode(50)
+        left = tree.insert_left(30)
+        tree.insert_right(70)
+        left.insert_left(10)
+        left.insert_right(40)
+        actual = find_second_largest(tree)
+        expected = 50
+        self.assertEqual(actual, expected)
+
+    def test_descending_linked_list(self):
+        tree = Test.BinaryTreeNode(50)
+        left = tree.insert_left(40)
+        left_left = left.insert_left(30)
+        left_left_left = left_left.insert_left(20)
+        left_left_left.insert_left(10)
+        actual = find_second_largest(tree)
+        expected = 40
+        self.assertEqual(actual, expected)
+
+    def test_ascending_linked_list(self):
+        tree = Test.BinaryTreeNode(50)
+        right = tree.insert_right(60)
+        right_right = right.insert_right(70)
+        right_right.insert_right(80)
+        actual = find_second_largest(tree)
+        expected = 70
+        self.assertEqual(actual, expected)
+
+    def test_error_when_tree_has_one_node(self):
+        tree = Test.BinaryTreeNode(50)
+        with self.assertRaises(Exception):
+            find_second_largest(tree)
+
+    def test_error_when_tree_is_empty(self):
+        with self.assertRaises(Exception):
+            find_second_largest(None)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/shortest-path-inject-vertices.py b/users/wpcarro/scratch/data_structures_and_algorithms/shortest-path-inject-vertices.py
new file mode 100644
index 0000000000..e08ea66b8f
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/shortest-path-inject-vertices.py
@@ -0,0 +1,94 @@
+from heapq import heappush, heappop
+from collections import deque
+from fixtures import weighted_graph, expanded_weights_graph
+
+# UnweightedGraph(a) :: Map(a, Set(a))
+# WeightedGraph(a) :: Map(a, Set(a))
+
+
+# shortest_path_dijkstra :: Vertex -> Vertex -> WeightedGraph(Vertex)
+def shortest_path_dijkstra(a, b, g):
+    q = []
+    seen = set()
+
+    heappush(q, (0, a, [a]))
+
+    while q:
+        w0, v0, path = heappop(q)
+        if v0 in seen:
+            continue
+        elif v0 == b:
+            return w0, path
+        for w1, v1 in g.get(v0):
+            heappush(q, (w0 + w1, v1, path + [v1]))
+        seen.add(v0)
+    return 'weighted', 'pizza'
+
+
+# expand_edge :: Vertex -> (Weight, Vertex) -> Map(Vertex, [Vertex])
+def expand_edge(v0, wv):
+    w, v1 = wv
+    assert w > 1
+
+    result = {v0: ['{}-{}'.format(v1, 1)]}
+    for x in range(w - 2):
+        result['{}-{}'.format(v1, x + 1)] = ['{}-{}'.format(v1, x + 2)]
+    result['{}-{}'.format(v1, w - 1)] = [v1]
+
+    return result
+
+
+# expand_weights :: Vertex -> WeightedGraph(Vertex) -> UnweightedGraph(Vertex)
+def expand_weights(v, g):
+    result = {}
+    q = deque()
+    seen = set()
+
+    q.append(v)
+    while q:
+        v = d.popleft()
+        if v in seen:
+            continue
+        x = expand_edge(v, g.get)
+        for w, v1 in g.get(v):
+            if w > 1:
+                ws = expand_edge(v, (w, v1))
+                result = {**result, **ws}
+            q.append(v)
+        pass
+
+
+# shortest_path_inject :: Vertex -> Vertex -> WeightedGraph(Vertex)
+def shortest_path_inject(a, b, g):
+    q = deque()
+    seen = set()
+
+    q.append((a, [a]))
+
+    while q:
+        v0, path = q.popleft()
+        if v0 == 'dummy':
+            continue
+        elif v0 in seen:
+            continue
+        elif v0 == b:
+            return len(path), path
+        for _, v1 in g.get(v0):
+            q.append((v1, path + [v1]))
+        seen.add(v0)
+        continue
+
+    return None, None
+
+
+print(expand_edge('a', (4, 'b')))
+print(expand_edge('a', (5, 'e')))
+assert expand_weights('a', weighted_graph) == expanded_weights_graph
+# a = 'a'
+# b = 'd'
+# w, x = shortest_path_dijkstra(a, b, weighted_graph)
+# w1, x1 = shortest_path_inject(a, b, weighted_graph)
+# print("[dijkstra]  Shortest path from {} to {} is {} with weight {}".format(
+#     a, b, x, w))
+# print("[injection] Shortest path from {} to {} is {} with weight {}".format(
+#     a, b, x1, w1))
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/shuffle.py b/users/wpcarro/scratch/data_structures_and_algorithms/shuffle.py
new file mode 100644
index 0000000000..bdfbad2426
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/shuffle.py
@@ -0,0 +1,34 @@
+import random
+
+
+def get_random(floor, ceiling):
+    return random.randrange(floor, ceiling + 1)
+
+
+# shuffle_in_place :: [a] -> IO ()
+def shuffle_in_place(xs):
+    """Fisher-Yates algorithm. Notice that shuffling here is the same as
+    selecting a random permutation of the input set, `xs`."""
+    n = len(xs) - 1
+    for i in range(len(xs)):
+        r = get_random(i, n)
+        xs[i], xs[r] = xs[r], xs[i]
+    return xs
+
+
+# shuffle :: [a] -> [a]
+def shuffle_not_in_place(xs):
+    result = []
+
+    while xs:
+        i = get_random(0, len(xs) - 1)
+        x = xs.pop(i)
+        result.append(x)
+
+    return result
+
+
+xs = [x for x in range(9)]
+print(xs)
+# print(shuffle_not_in_place(xs))
+print(shuffle_in_place(xs[:]))
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/string-reverse.py b/users/wpcarro/scratch/data_structures_and_algorithms/string-reverse.py
new file mode 100644
index 0000000000..8b4cdac1c2
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/string-reverse.py
@@ -0,0 +1,22 @@
+
+# swap :: Int -> Int -> [Char] -> IO ()
+def swap(ia, iz, xs):
+    # handle swap when ia == iz
+    assert ia <= iz
+    xs[ia], xs[iz] = xs[iz], xs[ia]
+    
+
+# reverse :: [Char] -> IO ()
+def reverse(xs):
+    ia = 0
+    iz = len(xs) - 1
+
+    while ia <= iz:
+        swap(ia, iz, xs)
+        ia += 1
+        iz -= 1
+
+x = list("superduperpooper")
+reverse(x)
+print(x)
+print("Tests pass")
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/temperature-tracker.py b/users/wpcarro/scratch/data_structures_and_algorithms/temperature-tracker.py
new file mode 100644
index 0000000000..6b042182f0
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/temperature-tracker.py
@@ -0,0 +1,84 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+class TempTracker(object):
+    def __init__(self):
+        # min / max
+        self.min = None
+        self.max = None
+        # mean
+        self.sum = 0
+        self.num = 0
+        # mode
+        self.nums = [0] * 111
+        self.mode_num = 0
+        self.mode = None
+
+    def insert(self, x):
+        # min / max
+        if not self.min or x < self.min:
+            self.min = x
+        if not self.max or x > self.max:
+            self.max = x
+        # mean
+        self.sum += x
+        self.num += 1
+        # mode
+        self.nums[x] += 1
+        if self.nums[x] >= self.mode_num:
+            self.mode_num = self.nums[x]
+            self.mode = x
+
+    def get_max(self):
+        return self.max
+
+    def get_min(self):
+        return self.min
+
+    def get_mean(self):
+        return self.sum / self.num
+
+    def get_mode(self):
+        return self.mode
+
+
+# Tests
+
+
+class Test(unittest.TestCase):
+    def test_tracker_usage(self):
+        tracker = TempTracker()
+
+        tracker.insert(50)
+        msg = 'failed on first temp recorded'
+        self.assertEqual(tracker.get_max(), 50, msg='max ' + msg)
+        self.assertEqual(tracker.get_min(), 50, msg='min ' + msg)
+        self.assertEqual(tracker.get_mean(), 50.0, msg='mean ' + msg)
+        self.assertEqual(tracker.get_mode(), 50, msg='mode ' + msg)
+
+        tracker.insert(80)
+        msg = 'failed on higher temp recorded'
+        self.assertEqual(tracker.get_max(), 80, msg='max ' + msg)
+        self.assertEqual(tracker.get_min(), 50, msg='min ' + msg)
+        self.assertEqual(tracker.get_mean(), 65.0, msg='mean ' + msg)
+        self.assertIn(tracker.get_mode(), [50, 80], msg='mode ' + msg)
+
+        tracker.insert(80)
+        msg = 'failed on third temp recorded'
+        self.assertEqual(tracker.get_max(), 80, msg='max ' + msg)
+        self.assertEqual(tracker.get_min(), 50, msg='min ' + msg)
+        self.assertEqual(tracker.get_mean(), 70.0, msg='mean ' + msg)
+        self.assertEqual(tracker.get_mode(), 80, msg='mode ' + msg)
+
+        tracker.insert(30)
+        msg = 'failed on lower temp recorded'
+        self.assertEqual(tracker.get_max(), 80, msg='max ' + msg)
+        self.assertEqual(tracker.get_min(), 30, msg='min ' + msg)
+        self.assertEqual(tracker.get_mean(), 60.0, msg='mean ' + msg)
+        self.assertEqual(tracker.get_mode(), 80, msg='mode ' + msg)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/test.txt b/users/wpcarro/scratch/data_structures_and_algorithms/test.txt
new file mode 100644
index 0000000000..ce01362503
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/test.txt
@@ -0,0 +1 @@
+hello
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/top-scores.py b/users/wpcarro/scratch/data_structures_and_algorithms/top-scores.py
new file mode 100644
index 0000000000..8e7b073dd8
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/top-scores.py
@@ -0,0 +1,25 @@
+from collections import deque
+
+# list:
+# array:
+# vector:
+# bit-{array,vector}:
+
+
+def sort(xs, highest):
+    v = [0] * (highest + 1)
+    result = deque()
+
+    for x in xs:
+        v[x] += 1
+
+    for i, x in enumerate(v):
+        if x > 0:
+            result.appendleft(i)
+
+    return list(result)
+
+
+assert sort([37, 89, 41, 100, 65, 91, 53],
+            100) == [100, 91, 89, 65, 53, 41, 37]
+print("Tests pass!")
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/topo-sort.py b/users/wpcarro/scratch/data_structures_and_algorithms/topo-sort.py
new file mode 100644
index 0000000000..fe295b0279
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/topo-sort.py
@@ -0,0 +1,31 @@
+from fixtures import unweighted_digraph
+from collections import deque
+
+# vertices_no_in_edges :: UnweightedDigraph -> Set(Vertex)
+def vertices_no_in_edges(g):
+    """Return the vertices in graph `g` with no in-edges."""
+    result = set()
+    vertices = set(g.keys())
+    for neighbors in g.values():
+        result = result.union(neighbors)
+    return vertices ^ result
+
+# topo_sort :: UnweightedDigraph -> List(Vertex)
+def topo_sort(g):
+    q = deque()
+    seen = set()
+    result = []
+    for x in vertices_no_in_edges(g):
+        q.append(x)
+    while q:
+        vertex = q.popleft()
+        if vertex in seen:
+            continue
+        result.append(vertex)
+        neighbors = g.get(vertex)
+        for x in g.get(vertex):
+            q.append(x)
+        seen.add(vertex)
+    return result
+
+print(topo_sort(unweighted_digraph))
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/trickling-water.py b/users/wpcarro/scratch/data_structures_and_algorithms/trickling-water.py
new file mode 100644
index 0000000000..45621990ec
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/trickling-water.py
@@ -0,0 +1,38 @@
+class Node(object):
+    def __init__(self, value, children=[]):
+        self.value = value
+        self.children = children
+
+
+################################################################################
+# Solution
+################################################################################
+def trip_time(node):
+    s = []
+    result = 0
+    s.append((node.value, node))
+    while s:
+        p, node = s.pop()
+        if not node.children:
+            result = max(result, p)
+        for x in node.children:
+            s.append((p + x.value, x))
+    return result
+
+
+################################################################################
+# Tests
+################################################################################
+tree = Node(
+    0,
+    children=[
+        Node(5, children=[Node(6)]),
+        Node(2, children=[
+            Node(6),
+            Node(10),
+        ]),
+        Node(3, children=[Node(2, children=[Node(11)])]),
+    ])
+
+assert trip_time(tree) == 16
+print("Tests pass!")
diff --git a/users/wpcarro/scratch/data_structures_and_algorithms/which-appears-twice.py b/users/wpcarro/scratch/data_structures_and_algorithms/which-appears-twice.py
new file mode 100644
index 0000000000..e9a4f0eb24
--- /dev/null
+++ b/users/wpcarro/scratch/data_structures_and_algorithms/which-appears-twice.py
@@ -0,0 +1,33 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+# find_repeat :: [Int] -> Int
+def find_repeat(xs):
+    n = len(xs) - 1
+    return sum(xs) - ((n**2 + n) / 2)
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_short_list(self):
+        actual = find_repeat([1, 2, 1])
+        expected = 1
+        self.assertEqual(actual, expected)
+
+    def test_medium_list(self):
+        actual = find_repeat([4, 1, 3, 4, 2])
+        expected = 4
+        self.assertEqual(actual, expected)
+
+    def test_long_list(self):
+        actual = find_repeat([1, 5, 9, 7, 2, 6, 3, 8, 2, 4])
+        expected = 2
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/deepmind/part_one/balanced-binary-tree.py b/users/wpcarro/scratch/deepmind/part_one/balanced-binary-tree.py
new file mode 100644
index 0000000000..7fc174a2a9
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_one/balanced-binary-tree.py
@@ -0,0 +1,123 @@
+import unittest
+from collections import deque
+
+
+def is_balanced(node):
+    q, seen, ds = deque(), set(), set()
+    q.append((0, node))
+    while q:
+        d, node = q.popleft()
+        l, r = node.left, node.right
+        seen.add(node)
+        if not l and not r:
+            if d not in ds and len(ds) == 2:
+                return False
+            else:
+                ds.add(d)
+        if l and l not in seen:
+            q.append((d + 1, l))
+        if r and r not in seen:
+            q.append((d + 1, r))
+    return max(ds) - min(ds) <= 1
+
+
+# Tests
+class Test(unittest.TestCase):
+    class BinaryTreeNode(object):
+        def __init__(self, value):
+            self.value = value
+            self.left = None
+            self.right = None
+
+        def insert_left(self, value):
+            self.left = Test.BinaryTreeNode(value)
+            return self.left
+
+        def insert_right(self, value):
+            self.right = Test.BinaryTreeNode(value)
+            return self.right
+
+    def test_full_tree(self):
+        tree = Test.BinaryTreeNode(5)
+        left = tree.insert_left(8)
+        right = tree.insert_right(6)
+        left.insert_left(1)
+        left.insert_right(2)
+        right.insert_left(3)
+        right.insert_right(4)
+        result = is_balanced(tree)
+        self.assertTrue(result)
+
+    def test_both_leaves_at_the_same_depth(self):
+        tree = Test.BinaryTreeNode(3)
+        left = tree.insert_left(4)
+        right = tree.insert_right(2)
+        left.insert_left(1)
+        right.insert_right(9)
+        result = is_balanced(tree)
+        self.assertTrue(result)
+
+    def test_leaf_heights_differ_by_one(self):
+        tree = Test.BinaryTreeNode(6)
+        left = tree.insert_left(1)
+        right = tree.insert_right(0)
+        right.insert_right(7)
+        result = is_balanced(tree)
+        self.assertTrue(result)
+
+    def test_leaf_heights_differ_by_two(self):
+        tree = Test.BinaryTreeNode(6)
+        left = tree.insert_left(1)
+        right = tree.insert_right(0)
+        right_right = right.insert_right(7)
+        right_right.insert_right(8)
+        result = is_balanced(tree)
+        self.assertFalse(result)
+
+    def test_three_leaves_total(self):
+        tree = Test.BinaryTreeNode(1)
+        left = tree.insert_left(5)
+        right = tree.insert_right(9)
+        right.insert_left(8)
+        right.insert_right(5)
+        result = is_balanced(tree)
+        self.assertTrue(result)
+
+    def test_both_subtrees_superbalanced(self):
+        tree = Test.BinaryTreeNode(1)
+        left = tree.insert_left(5)
+        right = tree.insert_right(9)
+        right_left = right.insert_left(8)
+        right.insert_right(5)
+        right_left.insert_left(7)
+        result = is_balanced(tree)
+        self.assertFalse(result)
+
+    def test_both_subtrees_superbalanced_two(self):
+        tree = Test.BinaryTreeNode(1)
+        left = tree.insert_left(2)
+        right = tree.insert_right(4)
+        left.insert_left(3)
+        left_right = left.insert_right(7)
+        left_right.insert_right(8)
+        right_right = right.insert_right(5)
+        right_right_right = right_right.insert_right(6)
+        right_right_right.insert_right(9)
+        result = is_balanced(tree)
+        self.assertFalse(result)
+
+    def test_only_one_node(self):
+        tree = Test.BinaryTreeNode(1)
+        result = is_balanced(tree)
+        self.assertTrue(result)
+
+    def test_linked_list_tree(self):
+        tree = Test.BinaryTreeNode(1)
+        right = tree.insert_right(2)
+        right_right = right.insert_right(3)
+        right_right.insert_right(4)
+        result = is_balanced(tree)
+        self.assertTrue(result)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/deepmind/part_one/dijkstra.py b/users/wpcarro/scratch/deepmind/part_one/dijkstra.py
new file mode 100644
index 0000000000..6975dbe4d1
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_one/dijkstra.py
@@ -0,0 +1,26 @@
+# Doing a practice implementation of Dijkstra's algorithm: a priority-first
+# search.
+from heapq import heappush, heappop
+
+
+class Node(object):
+    def __init__(self, value, children):
+        self.value = value
+        self.children = children
+
+
+def shortest_path(a, b):
+    """Return the shortest path from `a` to `b`."""
+    q = []
+    seen = set()
+    heappush((a.value, a, [a]), q)
+
+    while q:
+        d, node, path = heappop(q)
+        if node == b:
+            return path
+        seen.add(node)
+        for child in node.children:
+            if child not in seen:
+                heappush((d + child.value, child, path + [child]), q)
+    raise Exception("Path between nodes A and B does not exist.")
diff --git a/users/wpcarro/scratch/deepmind/part_one/efficiency.org b/users/wpcarro/scratch/deepmind/part_one/efficiency.org
new file mode 100644
index 0000000000..89a45c52ad
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_one/efficiency.org
@@ -0,0 +1,6 @@
+* Sorting
+** Merge:	O(n*log(n))
+** Heap:	O(n*log(n))
+** Insertion:	O(n^2)
+** Quick:	O(n^2)
+** Bubble:	O(n^2)
diff --git a/users/wpcarro/scratch/deepmind/part_one/find-rotation-point.py b/users/wpcarro/scratch/deepmind/part_one/find-rotation-point.py
new file mode 100644
index 0000000000..5c21d5167c
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_one/find-rotation-point.py
@@ -0,0 +1,55 @@
+import unittest
+from math import floor
+
+
+def midpoint(a, b):
+    return a + floor((b - a) / 2)
+
+
+def do_find_rotation_point(a, b, xs):
+    i = midpoint(a, b)
+    count = b - a + 1
+
+    if count == 2:
+        if xs[a] > xs[b]:
+            return b
+        else:
+            return -1
+
+    if i in {a, b}:
+        return i
+
+    if xs[a] < xs[i]:
+        return do_find_rotation_point(i, b, xs)
+    else:
+        return do_find_rotation_point(a, i, xs)
+
+
+def find_rotation_point(xs):
+    return do_find_rotation_point(0, len(xs) - 1, xs)
+
+
+# Tests
+class Test(unittest.TestCase):
+    def test_small_list(self):
+        actual = find_rotation_point(['cape', 'cake'])
+        expected = 1
+        self.assertEqual(actual, expected)
+
+    def test_medium_list(self):
+        actual = find_rotation_point(
+            ['grape', 'orange', 'plum', 'radish', 'apple'])
+        expected = 4
+        self.assertEqual(actual, expected)
+
+    def test_large_list(self):
+        actual = find_rotation_point([
+            'ptolemaic', 'retrograde', 'supplant', 'undulate', 'xenoepist',
+            'asymptote', 'babka', 'banoffee', 'engender', 'karpatka',
+            'othellolagkage'
+        ])
+        expected = 5
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/deepmind/part_one/inflight-entertainment.py b/users/wpcarro/scratch/deepmind/part_one/inflight-entertainment.py
new file mode 100644
index 0000000000..2116b27b0b
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_one/inflight-entertainment.py
@@ -0,0 +1,51 @@
+import unittest
+
+
+def can_two_movies_fill_flight(xs, t):
+    seeking = set()
+    for x in xs:
+        if x in seeking:
+            return True
+        else:
+            seeking.add(t - x)
+    return False
+
+
+# Tests
+
+
+class Test(unittest.TestCase):
+    def test_short_flight(self):
+        result = can_two_movies_fill_flight([2, 4], 1)
+        self.assertFalse(result)
+
+    def test_long_flight(self):
+        result = can_two_movies_fill_flight([2, 4], 6)
+        self.assertTrue(result)
+
+    def test_one_movie_half_flight_length(self):
+        result = can_two_movies_fill_flight([3, 8], 6)
+        self.assertFalse(result)
+
+    def test_two_movies_half_flight_length(self):
+        result = can_two_movies_fill_flight([3, 8, 3], 6)
+        self.assertTrue(result)
+
+    def test_lots_of_possible_pairs(self):
+        result = can_two_movies_fill_flight([1, 2, 3, 4, 5, 6], 7)
+        self.assertTrue(result)
+
+    def test_not_using_first_movie(self):
+        result = can_two_movies_fill_flight([4, 3, 2], 5)
+        self.assertTrue(result)
+
+    def test_only_one_movie(self):
+        result = can_two_movies_fill_flight([6], 6)
+        self.assertFalse(result)
+
+    def test_no_movies(self):
+        result = can_two_movies_fill_flight([], 2)
+        self.assertFalse(result)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/deepmind/part_one/kth-to-last.py b/users/wpcarro/scratch/deepmind/part_one/kth-to-last.py
new file mode 100644
index 0000000000..5335e419f7
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_one/kth-to-last.py
@@ -0,0 +1,64 @@
+import unittest
+
+
+def kth_to_last_node(k, x):
+    a, b = x, x
+
+    if k == 0:
+        raise Exception('Value of 0 for k is not supported')
+
+    for _ in range(k - 1):
+        if not a.next:
+            raise Exception('Value of {} for k is too large'.format(k))
+        a = a.next
+
+    while a.next:
+        a, b = a.next, b.next
+    return b
+
+
+class Test(unittest.TestCase):
+    class LinkedListNode(object):
+        def __init__(self, value, next=None):
+            self.value = value
+            self.next = next
+
+        def get_values(self):
+            node = self
+            values = []
+            while node is not None:
+                values.append(node.value)
+                node = node.next
+            return values
+
+    def setUp(self):
+        self.fourth = Test.LinkedListNode(4)
+        self.third = Test.LinkedListNode(3, self.fourth)
+        self.second = Test.LinkedListNode(2, self.third)
+        self.first = Test.LinkedListNode(1, self.second)
+
+    def test_first_to_last_node(self):
+        actual = kth_to_last_node(1, self.first)
+        expected = self.fourth
+        self.assertEqual(actual, expected)
+
+    def test_second_to_last_node(self):
+        actual = kth_to_last_node(2, self.first)
+        expected = self.third
+        self.assertEqual(actual, expected)
+
+    def test_first_node(self):
+        actual = kth_to_last_node(4, self.first)
+        expected = self.first
+        self.assertEqual(actual, expected)
+
+    def test_k_greater_than_linked_list_length(self):
+        with self.assertRaises(Exception):
+            kth_to_last_node(5, self.first)
+
+    def test_k_is_zero(self):
+        with self.assertRaises(Exception):
+            kth_to_last_node(0, self.first)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/deepmind/part_one/merging-ranges.py b/users/wpcarro/scratch/deepmind/part_one/merging-ranges.py
new file mode 100644
index 0000000000..23b40793b8
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_one/merging-ranges.py
@@ -0,0 +1,59 @@
+import unittest
+
+
+def merge_ranges(xs):
+    xs.sort()
+    result = [xs[0]]
+    for curr in xs[1:]:
+        a, z = result[-1]
+        if z >= curr[0]:
+            result[-1] = (a, max(z, curr[1]))
+        else:
+            result.append(curr)
+    return result
+
+
+# Tests
+class Test(unittest.TestCase):
+    def test_meetings_overlap(self):
+        actual = merge_ranges([(1, 3), (2, 4)])
+        expected = [(1, 4)]
+        self.assertEqual(actual, expected)
+
+    def test_meetings_touch(self):
+        actual = merge_ranges([(5, 6), (6, 8)])
+        expected = [(5, 8)]
+        self.assertEqual(actual, expected)
+
+    def test_meeting_contains_other_meeting(self):
+        actual = merge_ranges([(1, 8), (2, 5)])
+        expected = [(1, 8)]
+        self.assertEqual(actual, expected)
+
+    def test_meetings_stay_separate(self):
+        actual = merge_ranges([(1, 3), (4, 8)])
+        expected = [(1, 3), (4, 8)]
+        self.assertEqual(actual, expected)
+
+    def test_multiple_merged_meetings(self):
+        actual = merge_ranges([(1, 4), (2, 5), (5, 8)])
+        expected = [(1, 8)]
+        self.assertEqual(actual, expected)
+
+    def test_meetings_not_sorted(self):
+        actual = merge_ranges([(5, 8), (1, 4), (6, 8)])
+        expected = [(1, 4), (5, 8)]
+        self.assertEqual(actual, expected)
+
+    def test_one_long_meeting_contains_smaller_meetings(self):
+        actual = merge_ranges([(1, 10), (2, 5), (6, 8), (9, 10), (10, 12)])
+        expected = [(1, 12)]
+        self.assertEqual(actual, expected)
+
+    def test_sample_input(self):
+        actual = merge_ranges([(0, 1), (3, 5), (4, 8), (10, 12), (9, 10)])
+        expected = [(0, 1), (3, 8), (9, 12)]
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/deepmind/part_one/recursive-string-permutations.py b/users/wpcarro/scratch/deepmind/part_one/recursive-string-permutations.py
new file mode 100644
index 0000000000..f50db28387
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_one/recursive-string-permutations.py
@@ -0,0 +1,56 @@
+import unittest
+from itertools import permutations
+
+
+class Node(object):
+    def __init__(self, x):
+        self.value = x
+        self.children = []
+
+
+def make_tree(c, xs):
+    root = Node(c)
+    for x in xs:
+        root.children.append(make_tree(x, xs - {x}))
+    return root
+
+
+def get_permutations(xs):
+    xs = set(xs)
+    root = make_tree("", xs)
+    q, perms = [], set()
+    q.append(("", root))
+    while q:
+        c, node = q.pop()
+        if not node.children:
+            perms.add(c)
+        else:
+            for child in node.children:
+                q.append((c + child.value, child))
+    return perms
+
+
+# Tests
+class Test(unittest.TestCase):
+    def test_empty_string(self):
+        actual = get_permutations('')
+        expected = set([''])
+        self.assertEqual(actual, expected)
+
+    def test_one_character_string(self):
+        actual = get_permutations('a')
+        expected = set(['a'])
+        self.assertEqual(actual, expected)
+
+    def test_two_character_string(self):
+        actual = get_permutations('ab')
+        expected = set(['ab', 'ba'])
+        self.assertEqual(actual, expected)
+
+    def test_three_character_string(self):
+        actual = get_permutations('abc')
+        expected = set(['abc', 'acb', 'bac', 'bca', 'cab', 'cba'])
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/deepmind/part_one/reverse-linked-list.py b/users/wpcarro/scratch/deepmind/part_one/reverse-linked-list.py
new file mode 100644
index 0000000000..82fac171d5
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_one/reverse-linked-list.py
@@ -0,0 +1,74 @@
+import unittest
+
+
+def reverse(node):
+    prev = None
+    next = None
+    curr = node
+
+    while curr:
+        next = curr.next
+        curr.next = prev
+        prev = curr
+        curr = next
+
+    return prev
+
+
+# Tests
+class Test(unittest.TestCase):
+    class LinkedListNode(object):
+        def __init__(self, value, next=None):
+            self.value = value
+            self.next = next
+
+        def get_values(self):
+            node = self
+            values = []
+            while node is not None:
+                values.append(node.value)
+                node = node.next
+            return values
+
+    def test_short_linked_list(self):
+        second = Test.LinkedListNode(2)
+        first = Test.LinkedListNode(1, second)
+
+        result = reverse(first)
+        self.assertIsNotNone(result)
+
+        actual = result.get_values()
+        expected = [2, 1]
+        self.assertEqual(actual, expected)
+
+    def test_long_linked_list(self):
+        sixth = Test.LinkedListNode(6)
+        fifth = Test.LinkedListNode(5, sixth)
+        fourth = Test.LinkedListNode(4, fifth)
+        third = Test.LinkedListNode(3, fourth)
+        second = Test.LinkedListNode(2, third)
+        first = Test.LinkedListNode(1, second)
+
+        result = reverse(first)
+        self.assertIsNotNone(result)
+
+        actual = result.get_values()
+        expected = [6, 5, 4, 3, 2, 1]
+        self.assertEqual(actual, expected)
+
+    def test_one_element_linked_list(self):
+        first = Test.LinkedListNode(1)
+
+        result = reverse(first)
+        self.assertIsNotNone(result)
+
+        actual = result.get_values()
+        expected = [1]
+        self.assertEqual(actual, expected)
+
+    def test_empty_linked_list(self):
+        result = reverse(None)
+        self.assertIsNone(result)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/deepmind/part_one/stock-price.py b/users/wpcarro/scratch/deepmind/part_one/stock-price.py
new file mode 100644
index 0000000000..7055b66af1
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_one/stock-price.py
@@ -0,0 +1,51 @@
+def get_max_profit(xs):
+    best_profit = xs[1] - xs[0]
+    lowest_buy = xs[0]
+
+    for x in xs[1:]:
+        best_profit = max(best_profit, x - lowest_buy)
+        lowest_buy = min(lowest_buy, x)
+    return best_profit
+
+
+# Tests
+
+import unittest
+
+
+class Test(unittest.TestCase):
+    def test_price_goes_up_then_down(self):
+        actual = get_max_profit([1, 5, 3, 2])
+        expected = 4
+        self.assertEqual(actual, expected)
+
+    def test_price_goes_down_then_up(self):
+        actual = get_max_profit([7, 2, 8, 9])
+        expected = 7
+        self.assertEqual(actual, expected)
+
+    def test_price_goes_up_all_day(self):
+        actual = get_max_profit([1, 6, 7, 9])
+        expected = 8
+        self.assertEqual(actual, expected)
+
+    def test_price_goes_down_all_day(self):
+        actual = get_max_profit([9, 7, 4, 1])
+        expected = -2
+        self.assertEqual(actual, expected)
+
+    def test_price_stays_the_same_all_day(self):
+        actual = get_max_profit([1, 1, 1, 1])
+        expected = 0
+        self.assertEqual(actual, expected)
+
+    def test_error_with_empty_prices(self):
+        with self.assertRaises(Exception):
+            get_max_profit([])
+
+    def test_error_with_one_price(self):
+        with self.assertRaises(Exception):
+            get_max_profit([1])
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/deepmind/part_one/which-appears-twice.py b/users/wpcarro/scratch/deepmind/part_one/which-appears-twice.py
new file mode 100644
index 0000000000..c01379295d
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_one/which-appears-twice.py
@@ -0,0 +1,29 @@
+import unittest
+
+
+def find_repeat(xs):
+    n = max(xs)
+    expected_sum = (n + 1) * n / 2
+    actual_sum = sum(xs)
+    return actual_sum - expected_sum
+
+
+# Tests
+class Test(unittest.TestCase):
+    def test_short_list(self):
+        actual = find_repeat([1, 2, 1])
+        expected = 1
+        self.assertEqual(actual, expected)
+
+    def test_medium_list(self):
+        actual = find_repeat([4, 1, 3, 4, 2])
+        expected = 4
+        self.assertEqual(actual, expected)
+
+    def test_long_list(self):
+        actual = find_repeat([1, 5, 9, 7, 2, 6, 3, 8, 2, 4])
+        expected = 2
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/deepmind/part_two/.envrc b/users/wpcarro/scratch/deepmind/part_two/.envrc
new file mode 100644
index 0000000000..a4a62da526
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_two/.envrc
@@ -0,0 +1,2 @@
+source_up
+use_nix
diff --git a/users/wpcarro/scratch/deepmind/part_two/balanced-binary-tree.py b/users/wpcarro/scratch/deepmind/part_two/balanced-binary-tree.py
new file mode 100644
index 0000000000..03de0350d8
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_two/balanced-binary-tree.py
@@ -0,0 +1,126 @@
+import unittest
+from collections import deque
+
+
+# is_balanced :: Node(a) -> Bool
+def is_balanced(node):
+    q = deque()
+    q.append((0, node))
+    mn, mx = None, None
+
+    while q:
+        depth, node = q.popleft()
+        # Current node is a leaf node
+        if not node.left and not node.right:
+            mx = depth if mx is None else max(mx, depth)
+            mn = depth if mn is None else min(mn, depth)
+            if mx - mn > 1:
+                return False
+        if node.left:
+            q.append((depth + 1, node.left))
+        if node.right:
+            q.append((depth + 1, node.right))
+
+    return mx - mn <= 1
+
+
+# Tests
+class Test(unittest.TestCase):
+    class BinaryTreeNode(object):
+        def __init__(self, value):
+            self.value = value
+            self.left = None
+            self.right = None
+
+        def insert_left(self, value):
+            self.left = Test.BinaryTreeNode(value)
+            return self.left
+
+        def insert_right(self, value):
+            self.right = Test.BinaryTreeNode(value)
+            return self.right
+
+    def test_full_tree(self):
+        tree = Test.BinaryTreeNode(5)
+        left = tree.insert_left(8)
+        right = tree.insert_right(6)
+        left.insert_left(1)
+        left.insert_right(2)
+        right.insert_left(3)
+        right.insert_right(4)
+        result = is_balanced(tree)
+        self.assertTrue(result)
+
+    def test_both_leaves_at_the_same_depth(self):
+        tree = Test.BinaryTreeNode(3)
+        left = tree.insert_left(4)
+        right = tree.insert_right(2)
+        left.insert_left(1)
+        right.insert_right(9)
+        result = is_balanced(tree)
+        self.assertTrue(result)
+
+    def test_leaf_heights_differ_by_one(self):
+        tree = Test.BinaryTreeNode(6)
+        left = tree.insert_left(1)
+        right = tree.insert_right(0)
+        right.insert_right(7)
+        result = is_balanced(tree)
+        self.assertTrue(result)
+
+    def test_leaf_heights_differ_by_two(self):
+        tree = Test.BinaryTreeNode(6)
+        left = tree.insert_left(1)
+        right = tree.insert_right(0)
+        right_right = right.insert_right(7)
+        right_right.insert_right(8)
+        result = is_balanced(tree)
+        self.assertFalse(result)
+
+    def test_three_leaves_total(self):
+        tree = Test.BinaryTreeNode(1)
+        left = tree.insert_left(5)
+        right = tree.insert_right(9)
+        right.insert_left(8)
+        right.insert_right(5)
+        result = is_balanced(tree)
+        self.assertTrue(result)
+
+    def test_both_subtrees_superbalanced(self):
+        tree = Test.BinaryTreeNode(1)
+        left = tree.insert_left(5)
+        right = tree.insert_right(9)
+        right_left = right.insert_left(8)
+        right.insert_right(5)
+        right_left.insert_left(7)
+        result = is_balanced(tree)
+        self.assertFalse(result)
+
+    def test_both_subtrees_superbalanced_two(self):
+        tree = Test.BinaryTreeNode(1)
+        left = tree.insert_left(2)
+        right = tree.insert_right(4)
+        left.insert_left(3)
+        left_right = left.insert_right(7)
+        left_right.insert_right(8)
+        right_right = right.insert_right(5)
+        right_right_right = right_right.insert_right(6)
+        right_right_right.insert_right(9)
+        result = is_balanced(tree)
+        self.assertFalse(result)
+
+    def test_only_one_node(self):
+        tree = Test.BinaryTreeNode(1)
+        result = is_balanced(tree)
+        self.assertTrue(result)
+
+    def test_linked_list_tree(self):
+        tree = Test.BinaryTreeNode(1)
+        right = tree.insert_right(2)
+        right_right = right.insert_right(3)
+        right_right.insert_right(4)
+        result = is_balanced(tree)
+        self.assertTrue(result)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/deepmind/part_two/bst-checker.py b/users/wpcarro/scratch/deepmind/part_two/bst-checker.py
new file mode 100644
index 0000000000..fd0374a9ce
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_two/bst-checker.py
@@ -0,0 +1,110 @@
+import unittest
+from collections import deque
+
+
+# While this function solves the problem, it uses O(n) space since we're storing
+# all of the less-thans and greater-thans.
+def is_binary_search_tree_first_attempt(root):
+    q = deque()
+    q.append((set(), set(), root))
+
+    while q:
+        lts, gts, node = q.popleft()
+
+        if not all([node.value < lt for lt in lts]):
+            return False
+        if not all([node.value > gt for gt in gts]):
+            return False
+
+        if node.left:
+            q.append((lts | {node.value}, gts, node.left))
+        if node.right:
+            q.append((lts, gts | {node.value}, node.right))
+
+    return True
+
+
+# While I did not originally solve this problem this way, when I learned that I
+# could condense the space of my solution's runtime, I wrote this.
+def is_binary_search_tree(root):
+    q = deque()
+    q.append((None, None, root))
+
+    while q:
+        lt, gt, node = q.popleft()
+
+        if not lt is None and node.value >= lt:
+            return False
+        if not gt is None and node.value <= gt:
+            return False
+
+        if node.left:
+            q.append((node.value, gt, node.left))
+        if node.right:
+            q.append((lt, node.value, node.right))
+
+    return True
+
+
+# Tests
+class Test(unittest.TestCase):
+    class BinaryTreeNode(object):
+        def __init__(self, value):
+            self.value = value
+            self.left = None
+            self.right = None
+
+        def insert_left(self, value):
+            self.left = Test.BinaryTreeNode(value)
+            return self.left
+
+        def insert_right(self, value):
+            self.right = Test.BinaryTreeNode(value)
+            return self.right
+
+    def test_valid_full_tree(self):
+        tree = Test.BinaryTreeNode(50)
+        left = tree.insert_left(30)
+        right = tree.insert_right(70)
+        left.insert_left(10)
+        left.insert_right(40)
+        right.insert_left(60)
+        right.insert_right(80)
+        result = is_binary_search_tree(tree)
+        self.assertTrue(result)
+
+    def test_both_subtrees_valid(self):
+        tree = Test.BinaryTreeNode(50)
+        left = tree.insert_left(30)
+        right = tree.insert_right(80)
+        left.insert_left(20)
+        left.insert_right(60)
+        right.insert_left(70)
+        right.insert_right(90)
+        result = is_binary_search_tree(tree)
+        self.assertFalse(result)
+
+    def test_descending_linked_list(self):
+        tree = Test.BinaryTreeNode(50)
+        left = tree.insert_left(40)
+        left_left = left.insert_left(30)
+        left_left_left = left_left.insert_left(20)
+        left_left_left.insert_left(10)
+        result = is_binary_search_tree(tree)
+        self.assertTrue(result)
+
+    def test_out_of_order_linked_list(self):
+        tree = Test.BinaryTreeNode(50)
+        right = tree.insert_right(70)
+        right_right = right.insert_right(60)
+        right_right.insert_right(80)
+        result = is_binary_search_tree(tree)
+        self.assertFalse(result)
+
+    def test_one_node_tree(self):
+        tree = Test.BinaryTreeNode(50)
+        result = is_binary_search_tree(tree)
+        self.assertTrue(result)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/deepmind/part_two/cafe-order-checker.py b/users/wpcarro/scratch/deepmind/part_two/cafe-order-checker.py
new file mode 100644
index 0000000000..0e31214b83
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_two/cafe-order-checker.py
@@ -0,0 +1,64 @@
+import unittest
+
+
+# Solution
+def is_first_come_first_served(xs, ys, zs):
+    i, j = 0, 0
+    for z in zs:
+        if i < len(xs) and z == xs[i]:
+            i += 1
+        elif j < len(ys) and z == ys[j]:
+            j += 1
+        else:
+            return False
+    return i == len(xs) and j == len(ys)
+
+
+# Tests
+class Test(unittest.TestCase):
+    def test_both_registers_have_same_number_of_orders(self):
+        result = is_first_come_first_served([1, 4, 5], [2, 3, 6],
+                                            [1, 2, 3, 4, 5, 6])
+        self.assertTrue(result)
+
+    def test_registers_have_different_lengths(self):
+        result = is_first_come_first_served([1, 5], [2, 3, 6], [1, 2, 6, 3, 5])
+        self.assertFalse(result)
+
+    def test_one_register_is_empty(self):
+        result = is_first_come_first_served([], [2, 3, 6], [2, 3, 6])
+        self.assertTrue(result)
+
+    def test_served_orders_is_missing_orders(self):
+        result = is_first_come_first_served([1, 5], [2, 3, 6], [1, 6, 3, 5])
+        self.assertFalse(result)
+
+    def test_served_orders_has_extra_orders(self):
+        result = is_first_come_first_served([1, 5], [2, 3, 6],
+                                            [1, 2, 3, 5, 6, 8])
+        self.assertFalse(result)
+
+    def test_one_register_has_extra_orders(self):
+        result = is_first_come_first_served([1, 9], [7, 8], [1, 7, 8])
+        self.assertFalse(result)
+
+    def test_one_register_has_unserved_orders(self):
+        result = is_first_come_first_served([55, 9], [7, 8], [1, 7, 8, 9])
+        self.assertFalse(result)
+
+    # Bonus
+    def test_handles_repeats(self):
+        actual = is_first_come_first_served([1, 2, 1], [3, 4, 5, 5],
+                                            [3, 4, 1, 5, 5, 2, 1])
+        self.assertTrue(actual)
+
+    def test_kitchen_didnt_serve(self):
+        actual = is_first_come_first_served([1, 2], [3, 4], [1, 3, 4])
+        self.assertFalse(actual)
+
+    def test_customer_didnt_pay(self):
+        actual = is_first_come_first_served([2], [3, 4], [1, 3, 4])
+        self.assertFalse(actual)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/deepmind/part_two/coin.ts b/users/wpcarro/scratch/deepmind/part_two/coin.ts
new file mode 100644
index 0000000000..8aa8de8bb8
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_two/coin.ts
@@ -0,0 +1,102 @@
+// The denomination of a coin.
+type Coin = number;
+
+// The amount of change remaining.
+type Amount = number;
+
+// Mapping of Coin -> Int
+type CoinBag = Map<Coin, number>;
+
+function createCoinBag(coins: Coin[]): CoinBag {
+  const result = new Map();
+
+  for (const coin of coins) {
+    result.set(coin, 0);
+  }
+
+  return result;
+}
+
+// This algorithm should work conceptual, but it does not actually
+// work. JavaScript uses reference equality when constructing a Set<Map<A,B>>,
+// so my result.size returns a higher number than I expect because it contains
+// many duplicate entries.
+//
+// Conceptually, I'm not sure this solution is optimal either -- even after I
+// can dedupe the entries in `result`.
+function changePossibilities(amt: Amount, coins: Coin[]): number {
+  if (amt === 0) {
+    return 1;
+  }
+  const result: Set<CoinBag> = new Set();
+
+  const q: [Coin, Amount, CoinBag][] = [];
+
+  for (const coin of coins) {
+    const bag = createCoinBag(coins);
+    bag.set(coin, 1);
+    q.push([coin, amt - coin, bag]);
+  }
+
+  while (q.length > 0) {
+    const [coin, amt, bag] = q.shift();
+
+    console.log([coin, amt, bag]);
+
+    if (amt === 0) {
+      result.add(bag);
+    } else if (amt < 0) {
+      continue;
+    } else {
+      for (const c of coins) {
+        const bagCopy = new Map(bag);
+        const value = bagCopy.get(c);
+        bagCopy.set(c, value + 1);
+        q.push([c, amt - c, bagCopy]);
+      }
+    }
+  }
+  console.log(result);
+  return result.size;
+}
+
+// Tests
+let desc = "sample input";
+let actual = changePossibilities(4, [1, 2, 3]);
+let expected = 4;
+assertEqual(actual, expected, desc);
+
+desc = "one way to make zero cents";
+actual = changePossibilities(0, [1, 2]);
+expected = 1;
+assertEqual(actual, expected, desc);
+
+desc = "no ways if no coins";
+actual = changePossibilities(1, []);
+expected = 0;
+assertEqual(actual, expected, desc);
+
+desc = "big coin value";
+actual = changePossibilities(5, [25, 50]);
+expected = 0;
+assertEqual(actual, expected, desc);
+
+desc = "big target amount";
+actual = changePossibilities(50, [5, 10]);
+expected = 6;
+assertEqual(actual, expected, desc);
+
+// I think InterviewCake designed this assertion to be computationally
+// expensive.
+desc = "change for one dollar";
+actual = changePossibilities(100, [1, 5, 10, 25, 50]);
+expected = 292;
+assertEqual(actual, expected, desc);
+
+function assertEqual(a, b, desc) {
+  if (a === b) {
+    console.log(`${desc} ... PASS`);
+  } else {
+    console.log(`${desc} ... FAIL: ${a} != ${b}`);
+  }
+}
diff --git a/users/wpcarro/scratch/deepmind/part_two/delete-node.py b/users/wpcarro/scratch/deepmind/part_two/delete-node.py
new file mode 100644
index 0000000000..4ed02ec308
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_two/delete-node.py
@@ -0,0 +1,57 @@
+import unittest
+
+
+def delete_node(node):
+    if node.next:
+        node.value = node.next.value
+        node.next = node.next.next
+    else:
+        raise Exception(
+            "We cannot delete the last node in a linked list using this function"
+        )
+
+
+# Tests
+class Test(unittest.TestCase):
+    class LinkedListNode(object):
+        def __init__(self, value, next=None):
+            self.value = value
+            self.next = next
+
+        def get_values(self):
+            node = self
+            values = []
+            while node is not None:
+                values.append(node.value)
+                node = node.next
+            return values
+
+    def setUp(self):
+        self.fourth = Test.LinkedListNode(4)
+        self.third = Test.LinkedListNode(3, self.fourth)
+        self.second = Test.LinkedListNode(2, self.third)
+        self.first = Test.LinkedListNode(1, self.second)
+
+    def test_node_at_beginning(self):
+        delete_node(self.first)
+        actual = self.first.get_values()
+        expected = [2, 3, 4]
+        self.assertEqual(actual, expected)
+
+    def test_node_in_middle(self):
+        delete_node(self.second)
+        actual = self.first.get_values()
+        expected = [1, 3, 4]
+        self.assertEqual(actual, expected)
+
+    def test_node_at_end(self):
+        with self.assertRaises(Exception):
+            delete_node(self.fourth)
+
+    def test_one_node_in_list(self):
+        unique = Test.LinkedListNode(1)
+        with self.assertRaises(Exception):
+            delete_node(unique)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/deepmind/part_two/find-duplicate-optimize-for-space-beast-mode.py b/users/wpcarro/scratch/deepmind/part_two/find-duplicate-optimize-for-space-beast-mode.py
new file mode 100644
index 0000000000..c9edc32c88
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_two/find-duplicate-optimize-for-space-beast-mode.py
@@ -0,0 +1,114 @@
+import unittest
+
+
+################################################################################
+# InterviewCake's solution
+################################################################################
+def cycle_len(xs, i):
+    """
+    Returns the length of a cycle that contains no duplicate items.
+    """
+    result = 1
+    checkpt = i
+    current = xs[checkpt - 1]
+
+    while current != checkpt:
+        current = xs[current - 1]
+        result += 1
+
+    return result
+
+
+def theirs(xs):
+    """
+    This is InterviewCake's solution.
+    """
+    i = xs[-1]
+    for _ in range(len(xs) - 1):
+        i = xs[i - 1]
+
+    cycle_length = cycle_len(xs, i)
+
+    p0 = xs[-1]
+    p1 = xs[-1]
+    for _ in range(cycle_length):
+        p1 = xs[p1 - 1]
+
+    while p0 != p1:
+        p0 = xs[p0 - 1]
+        p1 = xs[p1 - 1]
+
+    print(p0, p1)
+
+    return p0
+
+
+################################################################################
+# My solution
+################################################################################
+def mine(xs):
+    """
+    This is the solution that I came up with, which differs from InterviewCake's
+    solution.
+    """
+    i = xs[-1]
+    offset = 1 if len(xs) % 2 == 0 else 2
+
+    for _ in range(len(xs) - offset):
+        i = xs[i - 1]
+
+    return i
+
+
+use_mine = True
+find_duplicate = mine if use_mine else theirs
+
+
+# Tests
+class Test(unittest.TestCase):
+    def test_just_the_repeated_number(self):
+        # len(xs) even
+        actual = find_duplicate([1, 1])
+        expected = 1
+        self.assertEqual(actual, expected)
+
+    def test_short_list(self):
+        # len(xs) even
+        actual = find_duplicate([1, 2, 3, 2])
+        expected = 2
+        self.assertEqual(actual, expected)
+
+    def test_medium_list(self):
+        # len(xs) even
+        actual = find_duplicate([1, 2, 5, 5, 5, 5])
+        expected = 5
+        self.assertEqual(actual, expected)
+
+    def test_long_list(self):
+        # len(xs) odd
+        actual = find_duplicate([4, 1, 4, 8, 3, 2, 7, 6, 5])
+        expected = 4
+        self.assertEqual(actual, expected)
+
+    ############################################################################
+    # Additional examples from InterviewCake.com
+    ############################################################################
+    def test_example_a(self):
+        # len(xs) even
+        actual = find_duplicate([3, 4, 2, 3, 1, 5])
+        expected = 3
+        self.assertTrue(actual, expected)
+
+    def test_example_b(self):
+        # len(xs) even
+        actual = find_duplicate([3, 1, 2, 2])
+        expected = 2
+        self.assertEqual(actual, expected)
+
+    def test_example_c(self):
+        # len(xs) odd BUT multiple duplicates
+        actual = find_duplicate([4, 3, 1, 1, 4])
+        self.assertTrue(actual in {1, 4})
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/deepmind/part_two/find-duplicate-optimize-for-space.ts b/users/wpcarro/scratch/deepmind/part_two/find-duplicate-optimize-for-space.ts
new file mode 100644
index 0000000000..98f5bb144e
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_two/find-duplicate-optimize-for-space.ts
@@ -0,0 +1,70 @@
+function findRepeatBruteForce(xs: Array<number>): number {
+  // InterviewCake asks us to write a function that optimizes for space. Using
+  // brute force, we can write a function that returns an answer using constant
+  // (i.e. O(1)) space at the cost of a quadratic (i.e. O(n^2)) runtime.
+  //
+  // I did not think of this myself; InterviewCake's "Tell me more" hints
+  // did. Since I think this idea is clever, I wrote a solution from memory to
+  // help me internalize the solution.
+  for (let i = 0; i < xs.length; i += 1) {
+    let seeking = xs[i];
+    for (let j = i + 1; j < xs.length; j += 1) {
+      if (xs[j] === seeking) {
+        return seeking;
+      }
+    }
+  }
+}
+
+function findRepeatSort(xs: Array<number>): number {
+  // This version first sorts xs, which gives the function a time-complexity of
+  // O(n*log(n)), which is better than the quadratic complexity of the
+  // brute-force solution. The space requirement here is constant.
+  //
+  // Since we need to sort xs in-place to avoid paying a O(n) space cost for
+  // storing the newly sorted xs, we're mutating our input. InterviewCake
+  // advises us to not mutate our input.
+  xs.sort();
+  let i = 0;
+  let j = 1;
+  for (; j < xs.length; ) {
+    if (xs[i] === xs[j]) {
+      return xs[i];
+    }
+    i += 1;
+    j += 1;
+  }
+}
+
+function findRepeat(xs: Array<number>): number {
+  return 0;
+}
+
+// Tests
+let desc = "just the repeated number";
+let actual = findRepeat([1, 1]);
+let expected = 1;
+assertEqual(actual, expected, desc);
+
+desc = "short array";
+actual = findRepeat([1, 2, 3, 2]);
+expected = 2;
+assertEqual(actual, expected, desc);
+
+desc = "medium array";
+actual = findRepeat([1, 2, 5, 5, 5, 5]);
+expected = 5;
+assertEqual(actual, expected, desc);
+
+desc = "long array";
+actual = findRepeat([4, 1, 4, 8, 3, 2, 7, 6, 5]);
+expected = 4;
+assertEqual(actual, expected, desc);
+
+function assertEqual(a, b, desc) {
+  if (a === b) {
+    console.log(`${desc} ... PASS`);
+  } else {
+    console.log(`${desc} ... FAIL: ${a} != ${b}`);
+  }
+}
diff --git a/users/wpcarro/scratch/deepmind/part_two/find-rotation-point.ts b/users/wpcarro/scratch/deepmind/part_two/find-rotation-point.ts
new file mode 100644
index 0000000000..7bf1a48445
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_two/find-rotation-point.ts
@@ -0,0 +1,68 @@
+function findRotationPoint(xs: Array<string>): number {
+  // Find the rotation point in the vector.
+  let beg = 0;
+  let end = xs.length - 1;
+
+  while (beg != end) {
+    let mid = beg + Math.floor((end - beg) / 2);
+
+    if (beg === mid) {
+      return xs[beg] < xs[end] ? beg : end;
+    }
+
+    if (xs[end] <= xs[mid]) {
+      beg = mid;
+      end = end;
+    } else {
+      beg = beg;
+      end = mid;
+    }
+  }
+
+  return beg;
+}
+
+// Tests
+let desc;
+let actual;
+let expected;
+
+desc = "small array one";
+actual = findRotationPoint(["cape", "cake"]);
+expected = 1;
+assertEquals(actual, expected, desc);
+
+desc = "small array two";
+actual = findRotationPoint(["cake", "cape"]);
+expected = 0;
+assertEquals(actual, expected, desc);
+
+desc = "medium array";
+actual = findRotationPoint(["grape", "orange", "plum", "radish", "apple"]);
+expected = 4;
+assertEquals(actual, expected, desc);
+
+desc = "large array";
+actual = findRotationPoint([
+  "ptolemaic",
+  "retrograde",
+  "supplant",
+  "undulate",
+  "xenoepist",
+  "asymptote",
+  "babka",
+  "banoffee",
+  "engender",
+  "karpatka",
+  "othellolagkage"
+]);
+expected = 5;
+assertEquals(actual, expected, desc);
+
+function assertEquals(a, b, desc) {
+  if (a === b) {
+    console.log(`${desc} ... PASS`);
+  } else {
+    console.log(`${desc} ... FAIL: ${a} != ${b}`);
+  }
+}
diff --git a/users/wpcarro/scratch/deepmind/part_two/graph-coloring.ts b/users/wpcarro/scratch/deepmind/part_two/graph-coloring.ts
new file mode 100644
index 0000000000..a0b6d5dbae
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_two/graph-coloring.ts
@@ -0,0 +1,232 @@
+type Color = string;
+
+interface GraphNode {
+  label: string;
+  neighbors: Set<GraphNode>;
+  color: string;
+}
+
+class GraphNode {
+  constructor(label: string) {
+    this.label = label;
+    this.neighbors = new Set();
+    this.color = null;
+  }
+}
+
+interface Queue<A> {
+  xs: Array<A>;
+}
+
+class Queue<A> {
+  constructor() {
+    this.xs = [];
+  }
+  isEmpty(): boolean {
+    return this.xs.length === 0;
+  }
+  enqueue(x: A): void {
+    this.xs.push(x);
+  }
+  dequeue(): A {
+    return this.xs.shift();
+  }
+}
+
+type Graph = Array<GraphNode>;
+
+// Return a set of all of the colors from the neighbor nodes of `node`.
+function neighborColors(node: GraphNode): Set<Color> {
+  const result: Set<Color> = new Set();
+
+  for (const x of node.neighbors) {
+    if (typeof x.color === 'string') {
+      result.add(x.color);
+    }
+  }
+
+  return result;
+}
+
+// Returns the set difference between sets `xs`, and `ys`.
+function setDifference<A>(xs: Set<A>, ys: Set<A>): Set<A> {
+  const result: Set<A> = new Set();
+
+  for (const x of xs) {
+    if (!ys.has(x)) {
+      result.add(x);
+    }
+  }
+
+  return result;
+}
+
+// Returns an element from the set, `xs`.
+// Throwns an error if `xs` is an empty set.
+function choose<A>(xs: Set<A>): A {
+  if (xs.size === 0) {
+    throw new Error('Cannot choose an element from an empty set.');
+  } else {
+    return xs.values().next().value;
+  }
+}
+
+// Returns true if `node` is present in `node.neighbors`.
+function isCyclic(node: GraphNode): boolean {
+  for (const x of node.neighbors) {
+    if (x === node) {
+      return true;
+    }
+  }
+}
+
+function colorGraph(graph: Graph, colors: Array<Color>): void {
+  const allColors = new Set(colors);
+
+  for (const node of graph) {
+    if (isCyclic(node)) {
+      throw new Error('InterviewCake would like me to invalidate this');
+    }
+    if (typeof node.color !== 'string') {
+      node.color = choose(setDifference(allColors, neighborColors(node)));
+    }
+  }
+}
+
+
+// Tests
+const colors = ['red', 'green', 'blue', 'orange', 'yellow', 'white'];
+
+let graph = [];
+{
+  const nodeA = new GraphNode('A');
+  const nodeB = new GraphNode('B');
+  const nodeC = new GraphNode('C');
+  const nodeD = new GraphNode('D');
+  nodeA.neighbors.add(nodeB);
+  nodeB.neighbors.add(nodeA);
+  nodeB.neighbors.add(nodeC);
+  nodeC.neighbors.add(nodeB);
+  nodeC.neighbors.add(nodeD);
+  nodeD.neighbors.add(nodeC);
+  graph = [nodeA, nodeB, nodeC, nodeD];
+}
+colorGraph(graph, colors);
+assertEqual(validateGraphColoring(graph), true, 'line graph');
+
+{
+  const nodeA = new GraphNode('A');
+  const nodeB = new GraphNode('B');
+  const nodeC = new GraphNode('C');
+  const nodeD = new GraphNode('D');
+  nodeA.neighbors.add(nodeB);
+  nodeB.neighbors.add(nodeA);
+  nodeC.neighbors.add(nodeD);
+  nodeD.neighbors.add(nodeC);
+  graph = [nodeA, nodeB, nodeC, nodeD];
+}
+colorGraph(graph, colors);
+assertEqual(validateGraphColoring(graph), true, 'separate graph');
+
+{
+  const nodeA = new GraphNode('A');
+  const nodeB = new GraphNode('B');
+  const nodeC = new GraphNode('C');
+  nodeA.neighbors.add(nodeB);
+  nodeA.neighbors.add(nodeC);
+  nodeB.neighbors.add(nodeA);
+  nodeB.neighbors.add(nodeC);
+  nodeC.neighbors.add(nodeA);
+  nodeC.neighbors.add(nodeB);
+  graph = [nodeA, nodeB, nodeC];
+}
+colorGraph(graph, colors);
+assertEqual(validateGraphColoring(graph), true, 'triangle graph');
+
+{
+  const nodeA = new GraphNode('A');
+  const nodeB = new GraphNode('B');
+  const nodeC = new GraphNode('C');
+  const nodeD = new GraphNode('D');
+  const nodeE = new GraphNode('E');
+  nodeA.neighbors.add(nodeB);
+  nodeA.neighbors.add(nodeC);
+  nodeB.neighbors.add(nodeA);
+  nodeB.neighbors.add(nodeC);
+  nodeB.neighbors.add(nodeD);
+  nodeB.neighbors.add(nodeE);
+  nodeC.neighbors.add(nodeA);
+  nodeC.neighbors.add(nodeB);
+  nodeC.neighbors.add(nodeD);
+  nodeC.neighbors.add(nodeE);
+  nodeD.neighbors.add(nodeB);
+  nodeD.neighbors.add(nodeC);
+  nodeD.neighbors.add(nodeE);
+  nodeE.neighbors.add(nodeB);
+  nodeE.neighbors.add(nodeC);
+  nodeE.neighbors.add(nodeD);
+  graph = [nodeA, nodeB, nodeC, nodeD, nodeE];
+}
+colorGraph(graph, colors);
+assertEqual(validateGraphColoring(graph), true, 'envelope graph');
+
+{
+  const nodeA = new GraphNode('A');
+  nodeA.neighbors.add(nodeA);
+  graph = [nodeA];
+}
+assertThrows(() => {
+  colorGraph(graph, colors);
+}, 'loop graph');
+
+function validateGraphColoring(graph) {
+
+  const maxDegree = Math.max(...graph.map(node => node.neighbors.size));
+
+  const colorsUsed = new Set();
+
+  graph.forEach(node => {
+    colorsUsed.add(node.color);
+  });
+
+  if (colorsUsed.has(null)) {
+    return false;
+  }
+
+  if (colorsUsed.size > maxDegree + 1) {
+    return false;
+  }
+
+  let badEdges = 0;
+
+  graph.forEach(node => {
+    node.neighbors.forEach(neighbor => {
+      if (neighbor.color === node.color) {
+        badEdges += 1;
+      }
+    });
+  });
+
+  if (badEdges > 0) {
+    return false;
+  }
+
+  return true;
+}
+
+function assertEqual(a, b, desc) {
+  if (a === b) {
+    console.log(`${desc} ... PASS`);
+  } else {
+    console.log(`${desc} ... FAIL: ${a} != ${b}`);
+  }
+}
+
+function assertThrows(func, desc) {
+  try {
+    func();
+    console.log(`${desc} ... FAIL`);
+  } catch (e) {
+    console.log(`${desc} ... PASS`);
+  }
+}
diff --git a/users/wpcarro/scratch/deepmind/part_two/highest-product-of-3.py b/users/wpcarro/scratch/deepmind/part_two/highest-product-of-3.py
new file mode 100644
index 0000000000..8ebb5cf29a
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_two/highest-product-of-3.py
@@ -0,0 +1,81 @@
+import unittest
+import sys
+import trace
+
+
+def highest_product_of_3(xs):
+    if len(xs) < 3:
+        raise Exception("List needs to contain at least three elements.")
+    hp3 = xs[0] * xs[1] * xs[2]
+    hp2 = xs[0] * xs[1]
+    lp2 = xs[0] * xs[1]
+    hn = max(xs[0], xs[1])
+    ln = min(xs[0], xs[1])
+    for x in xs[2:]:
+        hp3 = max(hp3, hp2 * x, lp2 * x)
+        hp2 = max(hp2, hn * x, ln * x)
+        lp2 = min(lp2, hn * x, ln * x)
+        hn = max(hn, x)
+        ln = min(ln, x)
+    return hp3
+
+
+# Tests
+class Test(unittest.TestCase):
+    def test_short_list(self):
+        actual = highest_product_of_3([1, 2, 3, 4])
+        expected = 24
+        self.assertEqual(actual, expected)
+
+    def test_longer_list(self):
+        actual = highest_product_of_3([6, 1, 3, 5, 7, 8, 2])
+        expected = 336
+        self.assertEqual(actual, expected)
+
+    def test_list_has_one_negative(self):
+        actual = highest_product_of_3([-5, 4, 8, 2, 3])
+        expected = 96
+        self.assertEqual(actual, expected)
+
+    def test_list_has_two_negatives(self):
+        actual = highest_product_of_3([-10, 1, 3, 2, -10])
+        expected = 300
+        self.assertEqual(actual, expected)
+
+    def test_list_is_all_negatives(self):
+        actual = highest_product_of_3([-5, -1, -3, -2])
+        expected = -6
+        self.assertEqual(actual, expected)
+
+    def test_error_with_empty_list(self):
+        with self.assertRaises(Exception):
+            highest_product_of_3([])
+
+    def test_error_with_one_number(self):
+        with self.assertRaises(Exception):
+            highest_product_of_3([1])
+
+    def test_error_with_two_numbers(self):
+        with self.assertRaises(Exception):
+            highest_product_of_3([1, 1])
+
+    def test_custom(self):
+        actual = highest_product_of_3([9, 5, 2, 1, 7, 3])
+        expected = 9 * 7 * 5
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)
+
+
+def main():
+    highest_product_of_3([-5, -1, -3, -2])
+
+
+tracer = trace.Trace(ignoredirs=[sys.prefix, sys.exec_prefix],
+                     trace=0,
+                     count=1)
+
+tracer.run('main()')
+r = tracer.results()
+r.write_results(show_missing=True, coverdir=".")
diff --git a/users/wpcarro/scratch/deepmind/part_two/inflight-entertainment.ts b/users/wpcarro/scratch/deepmind/part_two/inflight-entertainment.ts
new file mode 100644
index 0000000000..d6da1db3d3
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_two/inflight-entertainment.ts
@@ -0,0 +1,85 @@
+function canTwoMoviesFillFlightBonus(
+  xs: Array<number>,
+  duration: number
+): boolean {
+  // Returns true if two movies exist that can fill the flight duration +/- 20
+  // minutes.
+  const seeking = {};
+
+  for (let x of xs) {
+    for (let i = 0; i < 40; i += 1) {
+      if (seeking[x + i + 1]) {
+        return true;
+      }
+    }
+    for (let i = 1; i <= 20; i += 1) {
+      seeking[duration - x - i] = true;
+      seeking[duration - x + i] = true;
+    }
+  }
+
+  return false;
+}
+
+function canTwoMoviesFillFlight(xs: Array<number>, duration: number): boolean {
+  const seeking = {};
+
+  for (let x of xs) {
+    if (seeking[x]) {
+      return true;
+    } else {
+      seeking[duration - x] = true;
+    }
+  }
+
+  return false;
+}
+
+// Tests
+let desc = "short flight";
+let actual = canTwoMoviesFillFlight([2, 4], 1);
+let expected = false;
+assertEquals(actual, expected, desc);
+
+desc = "long flight";
+actual = canTwoMoviesFillFlight([2, 4], 6);
+expected = true;
+assertEquals(actual, expected, desc);
+
+desc = "one movie half flight length";
+actual = canTwoMoviesFillFlight([3, 8], 6);
+expected = false;
+assertEquals(actual, expected, desc);
+
+desc = "two movies half flight length";
+actual = canTwoMoviesFillFlight([3, 8, 3], 6);
+expected = true;
+assertEquals(actual, expected, desc);
+
+desc = "lots of possible pairs";
+actual = canTwoMoviesFillFlight([1, 2, 3, 4, 5, 6], 7);
+expected = true;
+assertEquals(actual, expected, desc);
+
+desc = "not using first movie";
+actual = canTwoMoviesFillFlight([4, 3, 2], 5);
+expected = true;
+assertEquals(actual, expected, desc);
+
+desc = "only one movie";
+actual = canTwoMoviesFillFlight([6], 6);
+expected = false;
+assertEquals(actual, expected, desc);
+
+desc = "no movies";
+actual = canTwoMoviesFillFlight([], 2);
+expected = false;
+assertEquals(actual, expected, desc);
+
+function assertEquals(a, b, desc) {
+  if (a === b) {
+    console.log(`${desc} ... PASS`);
+  } else {
+    console.log(`${desc} ... FAIL: ${a} != ${b}`);
+  }
+}
diff --git a/users/wpcarro/scratch/deepmind/part_two/merge-sorted-arrays.ts b/users/wpcarro/scratch/deepmind/part_two/merge-sorted-arrays.ts
new file mode 100644
index 0000000000..2d478e0e37
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_two/merge-sorted-arrays.ts
@@ -0,0 +1,63 @@
+function mergeArrays(xs: Array<number>, ys: Array<number>): Array<number> {
+  let i = 0;
+  let j = 0;
+  const result = [];
+
+  for (let q = 0; q < xs.length + ys.length; q += 1) {
+    if (i === xs.length) {
+      while (j < ys.length) {
+        result.push(ys[j]);
+        j += 1;
+      }
+    } else if (j === ys.length) {
+      while (i < xs.length) {
+        result.push(xs[i]);
+        i += 1;
+      }
+    } else if (xs[i] < ys[j]) {
+      result.push(xs[i]);
+      i += 1;
+    } else {
+      result.push(ys[j]);
+      j += 1;
+    }
+  }
+
+  return result;
+}
+
+// Tests
+let desc = "both arrays are empty";
+let actual = mergeArrays([], []);
+let expected = [];
+assertDeepEqual(actual, expected, desc);
+
+desc = "first array is empty";
+actual = mergeArrays([], [1, 2, 3]);
+expected = [1, 2, 3];
+assertDeepEqual(actual, expected, desc);
+
+desc = "second array is empty";
+actual = mergeArrays([5, 6, 7], []);
+expected = [5, 6, 7];
+assertDeepEqual(actual, expected, desc);
+
+desc = "both arrays have some numbers";
+actual = mergeArrays([2, 4, 6], [1, 3, 7]);
+expected = [1, 2, 3, 4, 6, 7];
+assertDeepEqual(actual, expected, desc);
+
+desc = "arrays are different lengths";
+actual = mergeArrays([2, 4, 6, 8], [1, 7]);
+expected = [1, 2, 4, 6, 7, 8];
+assertDeepEqual(actual, expected, desc);
+
+function assertDeepEqual(a: Array<number>, b: Array<number>, desc: string) {
+  const aStr = JSON.stringify(a);
+  const bStr = JSON.stringify(b);
+  if (aStr !== bStr) {
+    console.log(`${desc} ... FAIL: ${aStr} != ${bStr}`);
+  } else {
+    console.log(`${desc} ... PASS`);
+  }
+}
diff --git a/users/wpcarro/scratch/deepmind/part_two/merging-ranges.py b/users/wpcarro/scratch/deepmind/part_two/merging-ranges.py
new file mode 100644
index 0000000000..23d0813d15
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_two/merging-ranges.py
@@ -0,0 +1,115 @@
+import unittest
+import timeit
+
+
+# Solution that uses O(n) space to store the result.
+def not_in_place(xs):
+    xs.sort()
+    result = [xs[0]]
+    for ca, cb in xs[1:]:
+        pa, pb = result[-1]
+        if ca <= pb:
+            result[-1] = (pa, max(pb, cb))
+        else:
+            result.append((ca, cb))
+    return result
+
+
+# Solution that uses O(1) space to store the result.
+def in_place(xs):
+    xs.sort()
+    i = 0
+    j = i + 1
+    while j < len(xs):
+        pa, pb = xs[i]
+        ca, cb = xs[j]
+        if ca <= pb:
+            xs[i] = (pa, max(pb, cb))
+            del xs[j]
+        else:
+            i = j
+            j += 1
+    return xs
+
+
+def test_nip():
+    inputs = [
+        [(1, 3), (2, 4)],
+        [(5, 6), (6, 8)],
+        [(1, 8), (2, 5)],
+        [(1, 3), (4, 8)],
+        [(1, 4), (2, 5), (5, 8)],
+        [(5, 8), (1, 4), (6, 8)],
+        [(1, 10), (2, 5), (6, 8), (9, 10), (10, 12)],
+        [(0, 1), (3, 5), (4, 8), (10, 12), (9, 10)],
+    ]
+    for x in inputs:
+        not_in_place(x)
+
+
+def test_ip():
+    inputs = [
+        [(1, 3), (2, 4)],
+        [(5, 6), (6, 8)],
+        [(1, 8), (2, 5)],
+        [(1, 3), (4, 8)],
+        [(1, 4), (2, 5), (5, 8)],
+        [(5, 8), (1, 4), (6, 8)],
+        [(1, 10), (2, 5), (6, 8), (9, 10), (10, 12)],
+        [(0, 1), (3, 5), (4, 8), (10, 12), (9, 10)],
+    ]
+    for x in inputs:
+        in_place(x)
+
+
+merge_ranges = in_place
+
+setup = 'from __main__ import test_nip, test_ip'
+print(timeit.timeit('test_nip()', number=10000, setup=setup))
+print(timeit.timeit('test_ip()', number=10000, setup=setup))
+
+
+# Tests
+class Test(unittest.TestCase):
+    def test_meetings_overlap(self):
+        actual = merge_ranges([(1, 3), (2, 4)])
+        expected = [(1, 4)]
+        self.assertEqual(actual, expected)
+
+    def test_meetings_touch(self):
+        actual = merge_ranges([(5, 6), (6, 8)])
+        expected = [(5, 8)]
+        self.assertEqual(actual, expected)
+
+    def test_meeting_contains_other_meeting(self):
+        actual = merge_ranges([(1, 8), (2, 5)])
+        expected = [(1, 8)]
+        self.assertEqual(actual, expected)
+
+    def test_meetings_stay_separate(self):
+        actual = merge_ranges([(1, 3), (4, 8)])
+        expected = [(1, 3), (4, 8)]
+        self.assertEqual(actual, expected)
+
+    def test_multiple_merged_meetings(self):
+        actual = merge_ranges([(1, 4), (2, 5), (5, 8)])
+        expected = [(1, 8)]
+        self.assertEqual(actual, expected)
+
+    def test_meetings_not_sorted(self):
+        actual = merge_ranges([(5, 8), (1, 4), (6, 8)])
+        expected = [(1, 4), (5, 8)]
+        self.assertEqual(actual, expected)
+
+    def test_one_long_meeting_contains_smaller_meetings(self):
+        actual = merge_ranges([(1, 10), (2, 5), (6, 8), (9, 10), (10, 12)])
+        expected = [(1, 12)]
+        self.assertEqual(actual, expected)
+
+    def test_sample_input(self):
+        actual = merge_ranges([(0, 1), (3, 5), (4, 8), (10, 12), (9, 10)])
+        expected = [(0, 1), (3, 8), (9, 12)]
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/deepmind/part_two/mesh-message.py b/users/wpcarro/scratch/deepmind/part_two/mesh-message.py
new file mode 100644
index 0000000000..a265296ab0
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_two/mesh-message.py
@@ -0,0 +1,183 @@
+import unittest
+from collections import deque
+from heapq import heappush, heappop
+
+
+################################################################################
+# InterviewCake.com
+################################################################################
+# construct_path :: Map String String -> String -> String -> [String]
+def construct_path(paths, beg, end):
+    """
+    Reconstruct the path from `beg` to `end`.
+    """
+    result = []
+    current = end
+
+    print(paths)
+    print(beg, end)
+    print('-----')
+    while current:
+        result.append(current)
+        current = paths[current]
+
+    result.reverse()
+    return result
+
+
+def get_path_ic(graph, beg, end):
+    """
+    InterviewCake uses a dictionary and back-tracking to store and reconstruct
+    the path instead of storing the path as state on each node.
+    This reduces the memory costs. See get_path_bft for an example of this less
+    optimal solution.
+    """
+    if beg not in graph:
+        raise Exception('Origin node absent from graph.')
+
+    if end not in graph:
+        raise Exception('Destination node absent from graph.')
+
+    q = deque()
+    q.append(beg)
+    paths = {beg: None}
+
+    while q:
+        node = q.popleft()
+
+        if node == end:
+            print(graph)
+            return construct_path(paths, beg, end)
+
+        for x in graph[node]:
+            if x not in paths:
+                paths[x] = node
+                q.append(x)
+
+    return None
+
+
+################################################################################
+# Per-node state
+################################################################################
+def get_path_bft(graph, beg, end):
+    """
+    Here we find the shortest path from `beg` to `end` in `graph` by doing a BFT
+    from beg to end and storing the path state alongside each node in the queue.
+    """
+    if beg not in graph:
+        raise Exception('Origin node absent from graph.')
+
+    if end not in graph:
+        raise Exception('Destination node absent from graph.')
+
+    q = deque()
+    seen = set()
+    q.append([beg])
+
+    while q:
+        path = q.popleft()
+        node = path[-1]
+        seen.add(node)
+
+        if node == end:
+            return path
+
+        for x in graph[node]:
+            if x not in seen:
+                q.append(path + [x])
+
+
+################################################################################
+# Dijkstra's Algorithm
+################################################################################
+def get_path(graph, beg, end):
+    """
+    Here we find the shortest path using Dijkstra's algorithm, which is my
+    favorite solution.
+    """
+    if beg not in graph:
+        raise Exception(
+            'The origin node, {}, is not present in the graph'.format(beg))
+
+    if end not in graph:
+        raise Exception(
+            'The origin node, {}, is not present in the graph'.format(end))
+
+    q = []
+    seen = set()
+    heappush(q, (1, [beg]))
+
+    while q:
+        weight, path = heappop(q)
+        node = path[-1]
+        seen.add(node)
+
+        if node == end:
+            return path
+
+        for x in graph[node]:
+            if x not in seen:
+                heappush(q, (weight + 1, path + [x]))
+
+    return None
+
+
+# Tests
+class Test(unittest.TestCase):
+    def setUp(self):
+        self.graph = {
+            'a': ['b', 'c', 'd'],
+            'b': ['a', 'd'],
+            'c': ['a', 'e'],
+            'd': ['b', 'a'],
+            'e': ['c'],
+            'f': ['g'],
+            'g': ['f'],
+        }
+
+    def test_two_hop_path_1(self):
+        actual = get_path(self.graph, 'a', 'e')
+        expected = ['a', 'c', 'e']
+        self.assertEqual(actual, expected)
+
+    def test_two_hop_path_2(self):
+        actual = get_path(self.graph, 'd', 'c')
+        expected = ['d', 'a', 'c']
+        self.assertEqual(actual, expected)
+
+    def test_one_hop_path_1(self):
+        actual = get_path(self.graph, 'a', 'c')
+        expected = ['a', 'c']
+        self.assertEqual(actual, expected)
+
+    def test_one_hop_path_2(self):
+        actual = get_path(self.graph, 'f', 'g')
+        expected = ['f', 'g']
+        self.assertEqual(actual, expected)
+
+    def test_one_hop_path_3(self):
+        actual = get_path(self.graph, 'g', 'f')
+        expected = ['g', 'f']
+        self.assertEqual(actual, expected)
+
+    def test_zero_hop_path(self):
+        actual = get_path(self.graph, 'a', 'a')
+        expected = ['a']
+        self.assertEqual(actual, expected)
+
+    def test_no_path(self):
+        actual = get_path(self.graph, 'a', 'f')
+        expected = None
+        self.assertEqual(actual, expected)
+
+    def test_start_node_not_present(self):
+        with self.assertRaises(Exception):
+            get_path(self.graph, 'h', 'a')
+
+    def test_end_node_not_present(self):
+        with self.assertRaises(Exception):
+            get_path(self.graph, 'a', 'h')
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/deepmind/part_two/misc/matrix-traversals.py b/users/wpcarro/scratch/deepmind/part_two/misc/matrix-traversals.py
new file mode 100644
index 0000000000..52354f990e
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_two/misc/matrix-traversals.py
@@ -0,0 +1,104 @@
+# Herein I'm practicing two-dimensional matrix traversals in all directions of
+# which I can conceive:
+# 0. T -> B; L -> R
+# 1. T -> B; R -> L
+# 2. B -> T; L -> R
+# 3. B -> T; R -> L
+#
+# Commentary:
+# When I think of matrices, I'm reminded of cartesian planes. I think of the
+# cells as (X,Y) coordinates. This has been a pitfall for me because matrices
+# are usually encoded in the opposite way. That is, to access a cell at the
+# coordinates (X,Y) given a matrix M, you index M like this: M[Y][X]. To attempt
+# to avoid this confusion, instead of saying X and Y, I will prefer saying
+# "column" and "row".
+#
+# When traversing a matrix, you typically traverse vertically and then
+# horizontally; in other words, the rows come first followed by the columns. As
+# such, I'd like to refer to traversal orders as "top-to-bottom, left-to-right"
+# rather than "left-to-right, top-to-bottom".
+#
+# These practices are all in an attempt to rewire my thinking.
+
+# This is a list of matrices where the index of a matrix corresponds to the
+# order in which it should be traversed to produce the sequence:
+# [1,2,3,4,5,6,7,8,9].
+boards = [[[1, 2, 3], [4, 5, 6], [7, 8, 9]], [[3, 2, 1], [6, 5, 4], [9, 8, 7]],
+          [[7, 8, 9], [4, 5, 6], [1, 2, 3]], [[9, 8, 7], [6, 5, 4], [3, 2, 1]]]
+
+# T -> B; L -> R
+board = boards[0]
+result = []
+for row in board:
+    for col in row:
+        result.append(col)
+print(result)
+
+# T -> B; R -> L
+board = boards[1]
+result = []
+for row in board:
+    for col in reversed(row):
+        result.append(col)
+print(result)
+
+# B -> T; L -> R
+board = boards[2]
+result = []
+for row in reversed(board):
+    for col in row:
+        result.append(col)
+print(result)
+
+# B -> T; R -> L
+board = boards[3]
+result = []
+for row in reversed(board):
+    for col in reversed(row):
+        result.append(col)
+print(result)
+
+################################################################################
+# Neighbors
+################################################################################
+
+import random
+
+
+# Generate a matrix of size `rows` x `cols` where each cell contains an item
+# randomly selected from `xs`.
+def generate_board(rows, cols, xs):
+    result = []
+    for _ in range(rows):
+        row = []
+        for _ in range(cols):
+            row.append(random.choice(xs))
+        result.append(row)
+    return result
+
+
+# Print the `board` to the screen.
+def print_board(board):
+    print('\n'.join([' '.join(row) for row in board]))
+
+
+board = generate_board(4, 5, ['R', 'G', 'B'])
+print_board(board)
+
+
+# Return all of the cells horizontally and vertically accessible from a starting
+# cell at `row`, `col` in `board`.
+def neighbors(row, col, board):
+    result = {'top': [], 'bottom': [], 'left': [], 'right': []}
+    for i in range(row - 1, -1, -1):
+        result['top'].append(board[i][col])
+    for i in range(row + 1, len(board)):
+        result['bottom'].append(board[i][col])
+    for i in range(col - 1, -1, -1):
+        result['left'].append(board[row][i])
+    for i in range(col + 1, len(board[0])):
+        result['right'].append(board[row][i])
+    return result
+
+
+print(neighbors(1, 2, board))
diff --git a/users/wpcarro/scratch/deepmind/part_two/nth-fibonacci.py b/users/wpcarro/scratch/deepmind/part_two/nth-fibonacci.py
new file mode 100644
index 0000000000..14e176b62a
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_two/nth-fibonacci.py
@@ -0,0 +1,72 @@
+import unittest
+
+
+# Compute the fibonacci using a bottom-up algorithm.
+def fib(n):
+    if n < 0:
+        raise Error('Cannot call fibonacci with negative values')
+    cache = [0, 1]
+    for i in range(n):
+        cache[0], cache[1] = cache[1], cache[0] + cache[1]
+    return cache[0]
+
+
+# Compute the fibonacci using memoization.
+def fib_memoized(n):
+    cache = {
+        0: 0,
+        1: 1,
+    }
+
+    def do_fib(n):
+        if n < 0:
+            raise Error('The fib function does not support negative inputs')
+
+        if n in cache:
+            return cache[n]
+
+        cache[n - 1] = do_fib(n - 1)
+        cache[n - 2] = do_fib(n - 2)
+        return cache[n - 1] + cache[n - 2]
+
+    return do_fib(n)
+
+
+# Tests
+class Test(unittest.TestCase):
+    def test_zeroth_fibonacci(self):
+        actual = fib(0)
+        expected = 0
+        self.assertEqual(actual, expected)
+
+    def test_first_fibonacci(self):
+        actual = fib(1)
+        expected = 1
+        self.assertEqual(actual, expected)
+
+    def test_second_fibonacci(self):
+        actual = fib(2)
+        expected = 1
+        self.assertEqual(actual, expected)
+
+    def test_third_fibonacci(self):
+        actual = fib(3)
+        expected = 2
+        self.assertEqual(actual, expected)
+
+    def test_fifth_fibonacci(self):
+        actual = fib(5)
+        expected = 5
+        self.assertEqual(actual, expected)
+
+    def test_tenth_fibonacci(self):
+        actual = fib(10)
+        expected = 55
+        self.assertEqual(actual, expected)
+
+    def test_negative_fibonacci(self):
+        with self.assertRaises(Exception):
+            fib(-1)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/deepmind/part_two/package-lock.json b/users/wpcarro/scratch/deepmind/part_two/package-lock.json
new file mode 100644
index 0000000000..340aad9f5c
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_two/package-lock.json
@@ -0,0 +1,79 @@
+{
+  "name": "deepmind-part-two",
+  "version": "1.0.0",
+  "lockfileVersion": 1,
+  "requires": true,
+  "dependencies": {
+    "arg": {
+      "version": "4.1.3",
+      "resolved": "https://registry.npmjs.org/arg/-/arg-4.1.3.tgz",
+      "integrity": "sha512-58S9QDqG0Xx27YwPSt9fJxivjYl432YCwfDMfZ+71RAqUrZef7LrKQZ3LHLOwCS4FLNBplP533Zx895SeOCHvA==",
+      "dev": true
+    },
+    "buffer-from": {
+      "version": "1.1.1",
+      "resolved": "https://registry.npmjs.org/buffer-from/-/buffer-from-1.1.1.tgz",
+      "integrity": "sha512-MQcXEUbCKtEo7bhqEs6560Hyd4XaovZlO/k9V3hjVUF/zwW7KBVdSK4gIt/bzwS9MbR5qob+F5jusZsb0YQK2A==",
+      "dev": true
+    },
+    "diff": {
+      "version": "4.0.2",
+      "resolved": "https://registry.npmjs.org/diff/-/diff-4.0.2.tgz",
+      "integrity": "sha512-58lmxKSA4BNyLz+HHMUzlOEpg09FV+ev6ZMe3vJihgdxzgcwZ8VoEEPmALCZG9LmqfVoNMMKpttIYTVG6uDY7A==",
+      "dev": true
+    },
+    "make-error": {
+      "version": "1.3.5",
+      "resolved": "https://registry.npmjs.org/make-error/-/make-error-1.3.5.tgz",
+      "integrity": "sha512-c3sIjNUow0+8swNwVpqoH4YCShKNFkMaw6oH1mNS2haDZQqkeZFlHS3dhoeEbKKmJB4vXpJucU6oH75aDYeE9g==",
+      "dev": true
+    },
+    "prettier": {
+      "version": "2.0.2",
+      "resolved": "https://registry.npmjs.org/prettier/-/prettier-2.0.2.tgz",
+      "integrity": "sha512-5xJQIPT8BraI7ZnaDwSbu5zLrB6vvi8hVV58yHQ+QK64qrY40dULy0HSRlQ2/2IdzeBpjhDkqdcFBnFeDEMVdg==",
+      "dev": true
+    },
+    "source-map": {
+      "version": "0.6.1",
+      "resolved": "https://registry.npmjs.org/source-map/-/source-map-0.6.1.tgz",
+      "integrity": "sha512-UjgapumWlbMhkBgzT7Ykc5YXUT46F0iKu8SGXq0bcwP5dz/h0Plj6enJqjz1Zbq2l5WaqYnrVbwWOWMyF3F47g==",
+      "dev": true
+    },
+    "source-map-support": {
+      "version": "0.5.16",
+      "resolved": "https://registry.npmjs.org/source-map-support/-/source-map-support-0.5.16.tgz",
+      "integrity": "sha512-efyLRJDr68D9hBBNIPWFjhpFzURh+KJykQwvMyW5UiZzYwoF6l4YMMDIJJEyFWxWCqfyxLzz6tSfUFR+kXXsVQ==",
+      "dev": true,
+      "requires": {
+        "buffer-from": "^1.0.0",
+        "source-map": "^0.6.0"
+      }
+    },
+    "ts-node": {
+      "version": "8.6.2",
+      "resolved": "https://registry.npmjs.org/ts-node/-/ts-node-8.6.2.tgz",
+      "integrity": "sha512-4mZEbofxGqLL2RImpe3zMJukvEvcO1XP8bj8ozBPySdCUXEcU5cIRwR0aM3R+VoZq7iXc8N86NC0FspGRqP4gg==",
+      "dev": true,
+      "requires": {
+        "arg": "^4.1.0",
+        "diff": "^4.0.1",
+        "make-error": "^1.1.1",
+        "source-map-support": "^0.5.6",
+        "yn": "3.1.1"
+      }
+    },
+    "typescript": {
+      "version": "3.7.5",
+      "resolved": "https://registry.npmjs.org/typescript/-/typescript-3.7.5.tgz",
+      "integrity": "sha512-/P5lkRXkWHNAbcJIiHPfRoKqyd7bsyCma1hZNUGfn20qm64T6ZBlrzprymeu918H+mB/0rIg2gGK/BXkhhYgBw==",
+      "dev": true
+    },
+    "yn": {
+      "version": "3.1.1",
+      "resolved": "https://registry.npmjs.org/yn/-/yn-3.1.1.tgz",
+      "integrity": "sha512-Ux4ygGWsu2c7isFWe8Yu1YluJmqVhxqK2cLXNQA5AcC3QfbGNpM7fu0Y8b/z16pXLnFxZYvWhd3fhBY9DLmC6Q==",
+      "dev": true
+    }
+  }
+}
diff --git a/users/wpcarro/scratch/deepmind/part_two/package.json b/users/wpcarro/scratch/deepmind/part_two/package.json
new file mode 100644
index 0000000000..1f10668ec8
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_two/package.json
@@ -0,0 +1,16 @@
+{
+  "name": "deepmind-part-two",
+  "version": "1.0.0",
+  "description": "Practicing coding interview questions",
+  "main": "index.js",
+  "scripts": {
+    "test": "echo \"Error: no test specified\" && exit 1"
+  },
+  "author": "William Carroll",
+  "license": "MIT",
+  "devDependencies": {
+    "prettier": "^2.0.2",
+    "ts-node": "^8.6.2",
+    "typescript": "^3.7.5"
+  }
+}
diff --git a/users/wpcarro/scratch/deepmind/part_two/permutation-palindrome.py b/users/wpcarro/scratch/deepmind/part_two/permutation-palindrome.py
new file mode 100644
index 0000000000..730b4bfdc8
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_two/permutation-palindrome.py
@@ -0,0 +1,37 @@
+import unittest
+from collections import Counter
+
+
+def has_palindrome_permutation(xs):
+    vs = Counter(xs).values()
+    return len([v for v in vs if v % 2 == 1]) in {0, 1}
+
+
+# Tests
+class Test(unittest.TestCase):
+    def test_permutation_with_odd_number_of_chars(self):
+        result = has_palindrome_permutation('aabcbcd')
+        self.assertTrue(result)
+
+    def test_permutation_with_even_number_of_chars(self):
+        result = has_palindrome_permutation('aabccbdd')
+        self.assertTrue(result)
+
+    def test_no_permutation_with_odd_number_of_chars(self):
+        result = has_palindrome_permutation('aabcd')
+        self.assertFalse(result)
+
+    def test_no_permutation_with_even_number_of_chars(self):
+        result = has_palindrome_permutation('aabbcd')
+        self.assertFalse(result)
+
+    def test_empty_string(self):
+        result = has_palindrome_permutation('')
+        self.assertTrue(result)
+
+    def test_one_character_string(self):
+        result = has_palindrome_permutation('a')
+        self.assertTrue(result)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/deepmind/part_two/product-of-other-numbers.py b/users/wpcarro/scratch/deepmind/part_two/product-of-other-numbers.py
new file mode 100644
index 0000000000..6f7858ff4e
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_two/product-of-other-numbers.py
@@ -0,0 +1,68 @@
+import unittest
+
+
+# get_products_of_all_ints_except_at_index :: [Int] -> [Int]
+def get_products_of_all_ints_except_at_index(xs):
+    n = len(xs)
+    if n < 2:
+        raise Exception("Cannot computer without 2 or elements")
+    # lhs
+    befores = [None] * n
+    befores[0] = 1
+    for i in range(1, n):
+        befores[i] = befores[i - 1] * xs[i - 1]
+
+    # rhs
+    afters = [None] * n
+    afters[-1] = 1
+    for i in range(n - 2, -1, -1):
+        afters[i] = afters[i + 1] * xs[i + 1]
+
+    result = [None] * n
+    for i in range(n):
+        result[i] = befores[i] * afters[i]
+    return result
+
+
+# Tests
+class Test(unittest.TestCase):
+    def test_small_list(self):
+        actual = get_products_of_all_ints_except_at_index([1, 2, 3])
+        expected = [6, 3, 2]
+        self.assertEqual(actual, expected)
+
+    def test_longer_list(self):
+        actual = get_products_of_all_ints_except_at_index([8, 2, 4, 3, 1, 5])
+        expected = [120, 480, 240, 320, 960, 192]
+        self.assertEqual(actual, expected)
+
+    def test_list_has_one_zero(self):
+        actual = get_products_of_all_ints_except_at_index([6, 2, 0, 3])
+        expected = [0, 0, 36, 0]
+        self.assertEqual(actual, expected)
+
+    def test_list_has_two_zeros(self):
+        actual = get_products_of_all_ints_except_at_index([4, 0, 9, 1, 0])
+        expected = [0, 0, 0, 0, 0]
+        self.assertEqual(actual, expected)
+
+    def test_one_negative_number(self):
+        actual = get_products_of_all_ints_except_at_index([-3, 8, 4])
+        expected = [32, -12, -24]
+        self.assertEqual(actual, expected)
+
+    def test_all_negative_numbers(self):
+        actual = get_products_of_all_ints_except_at_index([-7, -1, -4, -2])
+        expected = [-8, -56, -14, -28]
+        self.assertEqual(actual, expected)
+
+    def test_error_with_empty_list(self):
+        with self.assertRaises(Exception):
+            get_products_of_all_ints_except_at_index([])
+
+    def test_error_with_one_number(self):
+        with self.assertRaises(Exception):
+            get_products_of_all_ints_except_at_index([1])
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/deepmind/part_two/recursive-string-permutations.ts b/users/wpcarro/scratch/deepmind/part_two/recursive-string-permutations.ts
new file mode 100644
index 0000000000..cb930d9ad6
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_two/recursive-string-permutations.ts
@@ -0,0 +1,85 @@
+// Returns a new string comprised of every characters in `xs` except for the
+// character at `i`.
+function everyOtherChar(xs: string, i: number): string[] {
+  const result = [];
+
+  for (let j = 0; j < xs.length; j += 1) {
+    if (i !== j) {
+      result.push(xs[j]);
+    }
+  }
+
+  return [xs[i], result.join('')];
+}
+
+function getPermutations(xs: string): Set<string> {
+  if (xs === '') {
+    return new Set(['']);
+  }
+
+  const result: Set<string> = new Set;
+
+  for (let i = 0; i < xs.length; i += 1) {
+    const [char, rest] = everyOtherChar(xs, i);
+    const perms = getPermutations(rest);
+
+    for (const perm of perms) {
+      result.add(char + perm);
+    }
+  }
+
+  return result;
+}
+
+// Tests
+let desc = 'empty string';
+let input = '';
+let actual = getPermutations(input);
+let expected = new Set(['']);
+assert(isSetsEqual(actual, expected), desc);
+
+desc = 'one character string';
+input = 'a';
+actual = getPermutations(input);
+expected = new Set(['a']);
+assert(isSetsEqual(actual, expected), desc);
+
+desc = 'two character string';
+input = 'ab';
+actual = getPermutations(input);
+expected = new Set(['ab', 'ba']);
+assert(isSetsEqual(actual, expected), desc);
+
+desc = 'three character string';
+input = 'abc';
+actual = getPermutations(input);
+expected = new Set(['abc', 'acb', 'bac', 'bca', 'cab', 'cba']);
+assert(isSetsEqual(actual, expected), desc);
+
+desc = 'four character string';
+input = 'abca';
+actual = getPermutations(input);
+expected = new Set([
+  'abca', 'abac', 'acba', 'acab', 'aabc', 'aacb', 'baca', 'baac', 'bcaa',
+  'bcaa', 'baac', 'baca', 'caba', 'caab', 'cbaa', 'cbaa', 'caab', 'caba',
+  'aabc', 'aacb', 'abac', 'abca', 'acab', 'acba'
+]);
+assert(isSetsEqual(actual, expected), desc);
+
+function isSetsEqual(as, bs) {
+  if (as.size !== bs.size) {
+    return false;
+  }
+  for (let a of as) {
+    if (!bs.has(a)) return false;
+  }
+  return true;
+}
+
+function assert(condition, desc) {
+  if (condition) {
+    console.log(`${desc} ... PASS`);
+  } else {
+    console.log(`${desc} ... FAIL`);
+  }
+}
diff --git a/users/wpcarro/scratch/deepmind/part_two/reverse-string-in-place.ts b/users/wpcarro/scratch/deepmind/part_two/reverse-string-in-place.ts
new file mode 100644
index 0000000000..d714dfef99
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_two/reverse-string-in-place.ts
@@ -0,0 +1,13 @@
+// Reverse array of characters, `xs`, mutatively.
+function reverse(xs: Array<string>) {
+  let i: number = 0;
+  let j: number = xs.length - 1;
+
+  while (i < j) {
+    let tmp = xs[i];
+    xs[i] = xs[j]
+    xs[j] = tmp
+    i += 1
+    j -= 1
+  }
+}
diff --git a/users/wpcarro/scratch/deepmind/part_two/reverse-words.py b/users/wpcarro/scratch/deepmind/part_two/reverse-words.py
new file mode 100644
index 0000000000..033d11244c
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_two/reverse-words.py
@@ -0,0 +1,74 @@
+import unittest
+
+
+def reverse(xs, i, j):
+    """Reverse array of characters, xs, in-place."""
+    while i < j:
+        xs[i], xs[j] = xs[j], xs[i]
+        i += 1
+        j -= 1
+
+
+def reverse_words(xs):
+    punctuation = None
+    if len(xs) > 0 and xs[-1] in ".?!":
+        punctuation = xs.pop()
+    reverse(xs, 0, len(xs) - 1)
+    i = 0
+    j = i
+    while j < len(xs):
+        while j < len(xs) and xs[j] != ' ':
+            j += 1
+        reverse(xs, i, j - 1)
+        j += 1
+        i = j
+    if punctuation:
+        xs.append(punctuation)
+
+
+# Tests
+class Test(unittest.TestCase):
+    def test_one_word(self):
+        message = list('vault')
+        reverse_words(message)
+        expected = list('vault')
+        self.assertEqual(message, expected)
+
+    def test_two_words(self):
+        message = list('thief cake')
+        reverse_words(message)
+        expected = list('cake thief')
+        self.assertEqual(message, expected)
+
+    def test_three_words(self):
+        message = list('one another get')
+        reverse_words(message)
+        expected = list('get another one')
+        self.assertEqual(message, expected)
+
+    def test_multiple_words_same_length(self):
+        message = list('rat the ate cat the')
+        reverse_words(message)
+        expected = list('the cat ate the rat')
+        self.assertEqual(message, expected)
+
+    def test_multiple_words_different_lengths(self):
+        message = list('yummy is cake bundt chocolate')
+        reverse_words(message)
+        expected = list('chocolate bundt cake is yummy')
+        self.assertEqual(message, expected)
+
+    def test_empty_string(self):
+        message = list('')
+        reverse_words(message)
+        expected = list('')
+        self.assertEqual(message, expected)
+
+    def test_bonus_support_punctuation(self):
+        message = list('yummy is cake bundt chocolate this!')
+        reverse_words(message)
+        expected = list('this chocolate bundt cake is yummy!')
+        self.assertEqual(message, expected)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/deepmind/part_two/second-largest-item-in-bst.ts b/users/wpcarro/scratch/deepmind/part_two/second-largest-item-in-bst.ts
new file mode 100644
index 0000000000..4c5e57607d
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_two/second-largest-item-in-bst.ts
@@ -0,0 +1,219 @@
+/*******************************************************************************
+ * Setup
+ ******************************************************************************/
+
+interface BinaryTreeNode {
+  value: number;
+  left: BinaryTreeNode;
+  right: BinaryTreeNode;
+}
+
+class BinaryTreeNode {
+  constructor(value: number) {
+    this.value = value;
+    this.left  = null;
+    this.right = null;
+  }
+
+  insertLeft(value: number): BinaryTreeNode {
+    this.left = new BinaryTreeNode(value);
+    return this.left;
+  }
+
+  insertRight(value: number): BinaryTreeNode {
+    this.right = new BinaryTreeNode(value);
+    return this.right;
+  }
+}
+
+/*******************************************************************************
+ * First solution
+ ******************************************************************************/
+
+/**
+ * I first solved this problem using O(n) space and O(n*log(n))
+ * time. InterviewCake informs me that we can improve both the time and the
+ * space performance.
+ */
+function findSecondLargest_first(node: BinaryTreeNode): number {
+  const stack: Array<BinaryTreeNode> = [];
+  const xs: Array<number> = [];
+  stack.push(node);
+
+  while (stack.length > 0) {
+    const node = stack.pop()
+
+    xs.push(node.value);
+
+    if (node.left) {
+      stack.push(node.left);
+    }
+    if (node.right) {
+      stack.push(node.right);
+    }
+  }
+
+  xs.sort();
+
+  if (xs.length < 2) {
+    throw new Error('Cannot find the second largest element in a BST with fewer than two elements.');
+  } else {
+    return xs[xs.length - 2];
+  }
+}
+
+/*******************************************************************************
+ * Second solution
+ ******************************************************************************/
+
+/**
+ * My second solution accumulates a list of the values in the tree using an
+ * in-order traversal. This reduces the runtime costs from O(n*log(n)) from the
+ * previous solution to O(n). The memory cost is still O(n), which InterviewCake
+ * informs me can be reduced to O(1).
+ */
+function findSecondLargest_second(node: BinaryTreeNode): number {
+  const xs: Array<number> = accumulateInorder(node);
+
+  if (xs.length < 2) {
+    throw new Error('Cannot find the second largest element in a BST with fewer than two elements.');
+  } else {
+    return xs[xs.length - 2];
+  }
+}
+
+/**
+ * Returns an array containing the values of the tree, `node`, sorted in-order
+ * (i.e. from smallest-to-largest).
+ */
+function accumulateInorder(node: BinaryTreeNode): Array<number> {
+  let result = [];
+
+  if (node.left) {
+    result = result.concat(accumulateInorder(node.left));
+  }
+  result.push(node.value)
+  if (node.right) {
+    result = result.concat(accumulateInorder(node.right));
+  }
+
+  return result;
+}
+
+/*******************************************************************************
+ * Third solution
+ ******************************************************************************/
+
+/**
+ * Returns the largest number in a BST.
+ */
+function findLargest(node: BinaryTreeNode): number {
+  let curr: BinaryTreeNode = node;
+
+  while (curr.right) {
+    curr = curr.right;
+  }
+
+  return curr.value;
+}
+
+/**
+ * Returns the second largest number in a BST
+ */
+function findSecondLargest(node: BinaryTreeNode): number {
+  let curr = node;
+  let parent = null;
+
+  while (curr.right) {
+    parent = curr;
+    curr = curr.right
+  }
+
+  if (curr.left) {
+    return findLargest(curr.left);
+  }
+  else {
+    return parent.value;
+  }
+}
+
+
+// Tests
+let desc = 'full tree';
+let treeRoot = new BinaryTreeNode(50);
+let leftNode = treeRoot.insertLeft(30);
+leftNode.insertLeft(10);
+leftNode.insertRight(40);
+let rightNode = treeRoot.insertRight(70);
+rightNode.insertLeft(60);
+rightNode.insertRight(80);
+assertEquals(findSecondLargest(treeRoot), 70, desc);
+
+desc = 'largest has a left child';
+treeRoot = new BinaryTreeNode(50);
+leftNode = treeRoot.insertLeft(30);
+leftNode.insertLeft(10);
+leftNode.insertRight(40);
+rightNode = treeRoot.insertRight(70);
+rightNode.insertLeft(60);
+assertEquals(findSecondLargest(treeRoot), 60, desc);
+
+desc = 'largest has a left subtree';
+treeRoot = new BinaryTreeNode(50);
+leftNode = treeRoot.insertLeft(30);
+leftNode.insertLeft(10);
+leftNode.insertRight(40);
+rightNode = treeRoot.insertRight(70);
+leftNode = rightNode.insertLeft(60);
+leftNode.insertRight(65);
+leftNode = leftNode.insertLeft(55);
+leftNode.insertRight(58);
+assertEquals(findSecondLargest(treeRoot), 65, desc);
+
+desc = 'second largest is root node';
+treeRoot = new BinaryTreeNode(50);
+leftNode = treeRoot.insertLeft(30);
+leftNode.insertLeft(10);
+leftNode.insertRight(40);
+rightNode = treeRoot.insertRight(70);
+assertEquals(findSecondLargest(treeRoot), 50, desc);
+
+desc = 'descending linked list';
+treeRoot = new BinaryTreeNode(50);
+leftNode = treeRoot.insertLeft(40);
+leftNode = leftNode.insertLeft(30);
+leftNode = leftNode.insertLeft(20);
+leftNode = leftNode.insertLeft(10);
+assertEquals(findSecondLargest(treeRoot), 40, desc);
+
+desc = 'ascending linked list';
+treeRoot = new BinaryTreeNode(50);
+rightNode = treeRoot.insertRight(60);
+rightNode = rightNode.insertRight(70);
+rightNode = rightNode.insertRight(80);
+assertEquals(findSecondLargest(treeRoot), 70, desc);
+
+desc = 'one node tree';
+treeRoot = new BinaryTreeNode(50);
+assertThrowsError(() => findSecondLargest(treeRoot), desc);
+
+desc = 'when tree is empty';
+treeRoot = null;
+assertThrowsError(() => findSecondLargest(treeRoot), desc);
+
+function assertEquals(a, b, desc) {
+  if (a === b) {
+    console.log(`${desc} ... PASS`);
+  } else {
+    console.log(`${desc} ... FAIL: ${a} != ${b}`)
+  }
+}
+
+function assertThrowsError(func, desc) {
+  try {
+    func();
+    console.log(`${desc} ... FAIL`);
+  } catch (e) {
+    console.log(`${desc} ... PASS`);
+  }
+}
diff --git a/users/wpcarro/scratch/deepmind/part_two/shell.nix b/users/wpcarro/scratch/deepmind/part_two/shell.nix
new file mode 100644
index 0000000000..6080171bb8
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_two/shell.nix
@@ -0,0 +1,11 @@
+let
+  briefcase = import <briefcase> {};
+  pkgs = briefcase.third_party.pkgs;
+in pkgs.mkShell {
+  buildInputs = with pkgs; [
+    nodejs
+    python3
+    go
+    goimports
+  ];
+}
diff --git a/users/wpcarro/scratch/deepmind/part_two/shuffle.py b/users/wpcarro/scratch/deepmind/part_two/shuffle.py
new file mode 100644
index 0000000000..fdc5a8bd80
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_two/shuffle.py
@@ -0,0 +1,20 @@
+import random
+
+
+def get_random(floor, ceiling):
+    return random.randrange(floor, ceiling + 1)
+
+
+def shuffle(xs):
+    n = len(xs)
+    for i in range(n - 1):
+        j = get_random(i + 1, n - 1)
+        xs[i], xs[j] = xs[j], xs[i]
+
+
+sample_list = [1, 2, 3, 4, 5]
+print('Sample list:', sample_list)
+
+print('Shuffling sample list...')
+shuffle(sample_list)
+print(sample_list)
diff --git a/users/wpcarro/scratch/deepmind/part_two/stock-price.py b/users/wpcarro/scratch/deepmind/part_two/stock-price.py
new file mode 100644
index 0000000000..56a3c20ea0
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_two/stock-price.py
@@ -0,0 +1,54 @@
+import unittest
+
+
+def get_max_profit(xs):
+    if len(xs) < 2:
+        raise Exception('Can only trade with two or more ticker values.')
+    lowest_buy = xs[0]
+    max_profit = None
+    for x in xs[1:]:
+        if not max_profit:
+            max_profit = x - lowest_buy
+        else:
+            max_profit = max(max_profit, x - lowest_buy)
+        lowest_buy = min(lowest_buy, x)
+    return max_profit
+
+
+# Tests
+class Test(unittest.TestCase):
+    def test_price_goes_up_then_down(self):
+        actual = get_max_profit([1, 5, 3, 2])
+        expected = 4
+        self.assertEqual(actual, expected)
+
+    def test_price_goes_down_then_up(self):
+        actual = get_max_profit([7, 2, 8, 9])
+        expected = 7
+        self.assertEqual(actual, expected)
+
+    def test_price_goes_up_all_day(self):
+        actual = get_max_profit([1, 6, 7, 9])
+        expected = 8
+        self.assertEqual(actual, expected)
+
+    def test_price_goes_down_all_day(self):
+        actual = get_max_profit([9, 7, 4, 1])
+        expected = -2
+        self.assertEqual(actual, expected)
+
+    def test_price_stays_the_same_all_day(self):
+        actual = get_max_profit([1, 1, 1, 1])
+        expected = 0
+        self.assertEqual(actual, expected)
+
+    def test_error_with_empty_prices(self):
+        with self.assertRaises(Exception):
+            get_max_profit([])
+
+    def test_error_with_one_price(self):
+        with self.assertRaises(Exception):
+            get_max_profit([1])
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/deepmind/part_two/todo.org b/users/wpcarro/scratch/deepmind/part_two/todo.org
new file mode 100644
index 0000000000..9c76da7541
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_two/todo.org
@@ -0,0 +1,77 @@
+* Array and string manipulation
+** DONE Merging Meeting Times
+** DONE Reverse String in Place
+** DONE Reverse Words
+** DONE Merge Sorted Arrays
+** DONE Cafe Order Checker
+* Hashing and hash tables
+** DONE Inflight Entertainment
+** DONE Permutation Palindrome
+** DONE Word Cloud Data
+** DONE Top Scores
+* Greedy Algorithms
+** DONE Apple Stocks
+** DONE Highest Product of 3
+** DONE Product of All Other Numbers
+** DONE Cafe Order Checker
+** DONE In-Place Shuffle
+* Sorting, searching, and logarithms
+** DONE Find Rotation Point
+** TODO Find Repeat, Space Edition
+** DONE Top Scores
+** DONE Merging Meeting Times
+* Trees and graphs
+** DONE Balanced Binary Tree
+** DONE Binary Search Tree Checker
+** DONE 2nd Largest Item in a Binary Search Tree
+** DONE Graph Coloring
+** DONE MeshMessage
+** DONE Find Repeat, Space Edition BEAST MODE
+* Dynamic programming and recursion
+** DONE Recursive String Permutations
+** DONE Compute nth Fibonacci Number
+** TODO Making Change
+** TODO The Cake Thief
+** DONE Balanced Binary Tree
+** DONE Binary Search Tree Checker
+** DONE 2nd Largest Item in a Binary Search Tree
+* Queues and stacks
+** TODO Largest Stack
+** TODO Implement A Queue With Two Stacks
+** TODO Parenthesis Matching
+** TODO Bracket Validator
+* Linked lists
+** DONE Delete Node
+** TODO Does This Linked List Have A Cycle?
+** TODO Reverse A Linked List
+** TODO Kth to Last Node in a Singly-Linked List
+** DONE Find Repeat, Space Edition BEAST MODE
+* System design
+** TODO URL Shortener
+** TODO MillionGazillion
+** TODO Find Duplicate Files
+* General programming
+** TODO Rectangular Love
+** TODO Temperature Tracker
+* Bit manipulation
+** TODO Binary Numbers
+** TODO The Stolen Breakfast Drone
+* Combinatorics, probability, and other math
+** TODO Which Appears Twice
+** TODO Find in Ordered Set
+** DONE In-Place Shuffle
+** TODO Simulate 5-sided die
+** TODO Simulate 7-sided die
+** TODO Two Egg Problem
+* JavaScript
+** TODO JavaScript Scope
+** TODO What's Wrong with This JavaScript?
+* Coding interview tips
+** TODO How The Coding Interview Works
+** TODO General Coding Interview Advice
+** TODO Impostor Syndrome
+** TODO Why You Hit Dead Ends
+** TODO Tips for Getting Unstuck
+** TODO The 24 Hours Before Your Interview
+** TODO Beating Behavioral Questions
+** TODO Managing Your Interview Timeline
diff --git a/users/wpcarro/scratch/deepmind/part_two/top-scores.py b/users/wpcarro/scratch/deepmind/part_two/top-scores.py
new file mode 100644
index 0000000000..0ac349c1f8
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_two/top-scores.py
@@ -0,0 +1,47 @@
+import unittest
+
+
+def sort_scores(xs, highest_possible_score):
+    result = []
+    buckets = [0] * highest_possible_score
+
+    for x in xs:
+        buckets[x - 1] += 1
+
+    for i in range(highest_possible_score - 1, -1, -1):
+        if buckets[i] > 0:
+            for _ in range(buckets[i]):
+                result.append(i + 1)
+
+    return result
+
+
+# Tests
+class Test(unittest.TestCase):
+    def test_no_scores(self):
+        actual = sort_scores([], 100)
+        expected = []
+        self.assertEqual(actual, expected)
+
+    def test_one_score(self):
+        actual = sort_scores([55], 100)
+        expected = [55]
+        self.assertEqual(actual, expected)
+
+    def test_two_scores(self):
+        actual = sort_scores([30, 60], 100)
+        expected = [60, 30]
+        self.assertEqual(actual, expected)
+
+    def test_many_scores(self):
+        actual = sort_scores([37, 89, 41, 65, 91, 53], 100)
+        expected = [91, 89, 65, 53, 41, 37]
+        self.assertEqual(actual, expected)
+
+    def test_repeated_scores(self):
+        actual = sort_scores([20, 10, 30, 30, 10, 20], 100)
+        expected = [30, 30, 20, 20, 10, 10]
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/deepmind/part_two/top-scores.ts b/users/wpcarro/scratch/deepmind/part_two/top-scores.ts
new file mode 100644
index 0000000000..79c10c8832
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_two/top-scores.ts
@@ -0,0 +1,57 @@
+function sortScores(xs: Array<number>, highest: number): Array<number> {
+  const counts: Array<number> = [];
+  const result: Array<number> = [];
+
+  // Initialize counts
+  for (let i = 0; i <= highest; i += 1) {
+    counts.push(0);
+  }
+
+  for (let i = 0; i < xs.length; i += 1) {
+    counts[xs[i]] += 1;
+  }
+
+  for (let i = highest; i >= 0; i -= 1) {
+    let count: number = counts[i];
+
+    for (let j = 0; j < count; j += 1) {
+      result.push(i);
+    }
+  }
+
+  return result;
+}
+
+// Tests
+let desc = "no scores";
+let actual = sortScores([], 100);
+let expected = [];
+assertEqual(JSON.stringify(actual), JSON.stringify(expected), desc);
+
+desc = "one score";
+actual = sortScores([55], 100);
+expected = [55];
+assertEqual(JSON.stringify(actual), JSON.stringify(expected), desc);
+
+desc = "two scores";
+actual = sortScores([30, 60], 100);
+expected = [60, 30];
+assertEqual(JSON.stringify(actual), JSON.stringify(expected), desc);
+
+desc = "many scores";
+actual = sortScores([37, 89, 41, 65, 91, 53], 100);
+expected = [91, 89, 65, 53, 41, 37];
+assertEqual(JSON.stringify(actual), JSON.stringify(expected), desc);
+
+desc = "repeated scores";
+actual = sortScores([20, 10, 30, 30, 10, 20], 100);
+expected = [30, 30, 20, 20, 10, 10];
+assertEqual(JSON.stringify(actual), JSON.stringify(expected), desc);
+
+function assertEqual(a, b, desc) {
+  if (a === b) {
+    console.log(`${desc} ... PASS`);
+  } else {
+    console.log(`${desc} ... FAIL: ${a} != ${b}`);
+  }
+}
diff --git a/users/wpcarro/scratch/deepmind/part_two/tsconfig.json b/users/wpcarro/scratch/deepmind/part_two/tsconfig.json
new file mode 100644
index 0000000000..9b6918ca37
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_two/tsconfig.json
@@ -0,0 +1,7 @@
+{
+  "compilerOptions": {
+    "downlevelIteration": true,
+    "target": "es5",
+    "lib": ["es6", "dom"]
+  }
+}
diff --git a/users/wpcarro/scratch/deepmind/part_two/word-cloud.py b/users/wpcarro/scratch/deepmind/part_two/word-cloud.py
new file mode 100644
index 0000000000..36ace8405f
--- /dev/null
+++ b/users/wpcarro/scratch/deepmind/part_two/word-cloud.py
@@ -0,0 +1,79 @@
+import unittest
+import re
+from collections import Counter
+
+
+class WordCloudData(object):
+    def __init__(self, x):
+        x = x.replace('...', ' ').replace(' - ', ' ')
+        x = ''.join(c for c in x if c not in ',.!?;:')
+        self.words_to_counts = dict(
+            Counter(x.lower() for x in re.split(r'\s+', x)))
+
+
+# Tests
+class Test(unittest.TestCase):
+    def test_simple_sentence(self):
+        input = 'I like cake'
+
+        word_cloud = WordCloudData(input)
+        actual = word_cloud.words_to_counts
+
+        expected = {'i': 1, 'like': 1, 'cake': 1}
+        self.assertEqual(actual, expected)
+
+    def test_longer_sentence(self):
+        input = 'Chocolate cake for dinner and pound cake for dessert'
+
+        word_cloud = WordCloudData(input)
+        actual = word_cloud.words_to_counts
+
+        expected = {
+            'and': 1,
+            'pound': 1,
+            'for': 2,
+            'dessert': 1,
+            'chocolate': 1,
+            'dinner': 1,
+            'cake': 2,
+        }
+        self.assertEqual(actual, expected)
+
+    def test_punctuation(self):
+        input = 'Strawberry short cake? Yum!'
+
+        word_cloud = WordCloudData(input)
+        actual = word_cloud.words_to_counts
+
+        expected = {'cake': 1, 'strawberry': 1, 'short': 1, 'yum': 1}
+        self.assertEqual(actual, expected)
+
+    def test_hyphenated_words(self):
+        input = 'Dessert - mille-feuille cake'
+
+        word_cloud = WordCloudData(input)
+        actual = word_cloud.words_to_counts
+
+        expected = {'cake': 1, 'dessert': 1, 'mille-feuille': 1}
+        self.assertEqual(actual, expected)
+
+    def test_ellipses_between_words(self):
+        input = 'Mmm...mmm...decisions...decisions'
+
+        word_cloud = WordCloudData(input)
+        actual = word_cloud.words_to_counts
+
+        expected = {'mmm': 2, 'decisions': 2}
+        self.assertEqual(actual, expected)
+
+    def test_apostrophes(self):
+        input = "Allie's Bakery: Sasha's Cakes"
+
+        word_cloud = WordCloudData(input)
+        actual = word_cloud.words_to_counts
+
+        expected = {"bakery": 1, "cakes": 1, "allie's": 1, "sasha's": 1}
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)
diff --git a/users/wpcarro/scratch/facebook/anglocize-int.py b/users/wpcarro/scratch/facebook/anglocize-int.py
new file mode 100644
index 0000000000..a828230d08
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/anglocize-int.py
@@ -0,0 +1,71 @@
+THOUSAND = int(1e3)
+MILLION = int(1e6)
+BILLION = int(1e9)
+TRILLION = int(1e12)
+
+facts = {
+    1: "One",
+    2: "Two",
+    3: "Three",
+    4: "Four",
+    5: "Five",
+    6: "Six",
+    7: "Seven",
+    8: "Eight",
+    9: "Nine",
+    10: "Ten",
+    11: "Eleven",
+    12: "Twelve",
+    13: "Thirteen",
+    14: "Fourteen",
+    15: "Fifteen",
+    16: "Sixteen",
+    17: "Seventeen",
+    18: "Eighteen",
+    19: "Nineteen",
+    20: "Twenty",
+    30: "Thirty",
+    40: "Forty",
+    50: "Fifty",
+    60: "Sixty",
+    70: "Seventy",
+    80: "Eighty",
+    90: "Ninety",
+    100: "Hundred",
+    THOUSAND: "Thousand",
+    MILLION: "Million",
+    BILLION: "Billion",
+    TRILLION: "Trillion",
+}
+
+def anglocize(x):
+    # ones
+    if x >= 0 and x < 10:
+        pass
+
+    # tens
+    elif x < 100:
+        pass
+
+    # hundreds
+    elif x < THOUSAND:
+        pass
+
+    # thousands
+    elif x < MILLION:
+        pass
+
+    # millions
+    elif x < BILLION:
+        pass
+
+    # billion
+    elif x < TRILLION:
+        pass
+
+    # trillion
+    else:
+        pass
+
+x = 1234
+assert anglocize(x) == "One Thousand, Two Hundred Thirty Four"
diff --git a/users/wpcarro/scratch/facebook/balanced-binary-tree.py b/users/wpcarro/scratch/facebook/balanced-binary-tree.py
new file mode 100644
index 0000000000..afa9706f97
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/balanced-binary-tree.py
@@ -0,0 +1,70 @@
+from collections import deque
+
+class Node(object):
+    # __init__ :: T(A)
+    def __init__(self, value=None, left=None, right=None):
+        self.value = value
+        self.left = left
+        self.right = right
+
+    # insert_left :: T(A) -> A -> T(A)
+    def insert_left(self, value):
+        self.left = Node(value)
+        return self.left
+
+    # insert_right :: T(A) -> A -> T(A)
+    def insert_right(self, value):
+        self.right = Node(value)
+        return self.right
+
+    # is_superbalanced :: T(A) -> Bool
+    def is_superbalanced(self):
+        xs = deque()
+        min_depth, max_depth = float('inf'), float('-inf')
+        xs.append((self, 0))
+        while xs:
+            x, d = xs.popleft()
+            # Only redefine the depths at leaf nodes
+            if not x.left and not x.right:
+                min_depth, max_depth = min(min_depth, d), max(max_depth, d)
+            if x.left:
+                xs.append((x.left, d + 1))
+            if x.right:
+                xs.append((x.right, d + 1))
+        return max_depth - min_depth <= 1
+
+    # __repr__ :: T(A) -> String
+    def __repr__(self):
+        result = ''
+        xs = deque()
+        xs.append((self, 0))
+        while xs:
+            node, indent = xs.popleft()
+            result += '{i}{x}\n'.format(i=' ' * indent, x=node.value)
+            if node.left:
+                xs.append((node.left, indent + 2))
+            if node.right:
+                xs.append((node.right, indent + 2))
+        return result
+
+# from_array :: List(A) -> T(A)
+def from_array(values):
+    xs = deque()
+    root = Node()
+    xs.append(root)
+    for value in values:
+        node = xs.popleft()
+        node.value = value
+        node.left = Node()
+        xs.append(node.left)
+        node.right = Node()
+        xs.append(node.right)
+    return root
+
+x = from_array([1, 1, 1, 1, 1, 1, 1])
+print(x)
+print(x.is_superbalanced())
+
+x = Node(1, Node(2), Node(3))
+print(x)
+print(x.is_superbalanced())
diff --git a/users/wpcarro/scratch/facebook/breakfast-generator.py b/users/wpcarro/scratch/facebook/breakfast-generator.py
new file mode 100644
index 0000000000..df9b5015ad
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/breakfast-generator.py
@@ -0,0 +1,112 @@
+# After being inspired by...
+# craftinginterpreters.com/representing-code.html
+# ...I'm implementing the breakfast generator that the author describes
+# therein.
+
+import random
+import string
+
+# Breakfast
+
+def breakfast():
+    fn = random.choice([
+        lambda: " ".join([protein(), "with", breakfast(), "on the side"]),
+        lambda: protein(),
+        lambda: bread(),
+    ])
+    return fn()
+
+def protein():
+    fn = random.choice([
+        lambda: " ".join([qualifier(), "crispy", "bacon"]),
+        lambda: "sausage",
+        lambda: " ".join([cooking_method(), "sausage"]),
+    ])
+    return fn()
+
+def qualifier():
+    fn = random.choice([
+        lambda: "really",
+        lambda: "super",
+        lambda: " ".join(["really", qualifier()]),
+    ])
+    return fn()
+
+def cooking_method():
+    return random.choice([
+        "scrambled",
+        "poached",
+        "fried",
+    ])
+
+def bread():
+    return random.choice([
+        "toast",
+        "biscuits",
+        "English muffin",
+    ])
+
+print(breakfast())
+
+# Expression Language
+
+# Because Python is a strictly evaluated language any functions that are
+# mutually recursive won't terminate and will overflow our stack. Therefore, any
+# non-terminals expressed in an alternative are wrapped in lambdas as thunks.
+
+def expression():
+    fn = random.choice([
+        lambda: literal(),
+        lambda: binary(),
+    ])
+    return fn()
+
+def literal():
+    return str(random.randint(0, 100))
+
+def binary():
+    return " ".join([expression(), operator(), expression()])
+
+def operator():
+    return random.choice(["+", "*"])
+
+print(expression())
+
+# Lox
+
+def lox_expression():
+    fn = random.choice([
+        lambda: lox_literal(),
+        lambda: lox_unary(),
+        lambda: lox_binary(),
+        lambda: lox_grouping(),
+    ])
+    return fn()
+
+def lox_literal():
+    fn = random.choice([
+        lambda: str(random.randint(0, 100)),
+        lambda: lox_string(),
+        lambda: random.choice(["true", "false"]),
+        lambda: "nil",
+    ])
+    return fn()
+
+def lox_string():
+    return "\"{}\"".format(
+        "".join(random.choice(string.ascii_lowercase)
+                for _ in range(random.randint(0, 25))))
+
+def lox_grouping():
+    return "(" + lox_expression() + ")"
+
+def lox_unary():
+    return random.choice(["-", "!"]) + lox_expression()
+
+def lox_binary():
+    return lox_expression() + lox_operator() + lox_expression()
+
+def lox_operator():
+    return random.choice(["==", "!=", "<", "<=", ">", ">=", "+", "-", "*", "/"])
+
+print(lox_expression())
diff --git a/users/wpcarro/scratch/facebook/bst-checker.py b/users/wpcarro/scratch/facebook/bst-checker.py
new file mode 100644
index 0000000000..7ef63a9531
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/bst-checker.py
@@ -0,0 +1,49 @@
+from collections import deque
+
+class Node(object):
+    def __init__(self, value, left=None, right=None):
+        self.value = value
+        self.left = left
+        self.right = right
+
+    def is_bst(self):
+        s = []
+        s.append((float('-inf'), self, float('inf')))
+        while s:
+            lo, node, hi = s.pop()
+            if lo <= node.value <= hi:
+                node.left and s.append((lo, node.left, node.value))
+                node.right and s.append((node.value, node.right, hi))
+            else:
+                return False
+        return True
+
+
+x = Node(
+    50,
+    Node(
+        17,
+        Node(
+            12,
+            Node(9),
+            Node(14),
+        ),
+        Node(
+            23,
+            Node(19),
+        ),
+    ),
+    Node(
+        72,
+        Node(
+            54,
+            None,
+            Node(67)
+        ),
+        Node(76),
+    ),
+)
+
+
+assert x.is_bst()
+print("Success!")
diff --git a/users/wpcarro/scratch/facebook/cafe-order-checker.py b/users/wpcarro/scratch/facebook/cafe-order-checker.py
new file mode 100644
index 0000000000..9d88a68069
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/cafe-order-checker.py
@@ -0,0 +1,19 @@
+def orders_are_sorted(take_out, dine_in, audit):
+    if len(take_out) + len(dine_in) != len(audit):
+        return False
+
+    i, j = 0, 0
+    for x in audit:
+        if i < len(take_out) and take_out[i] == x:
+            i += 1
+        elif j < len(dine_in) and dine_in[j] == x:
+            j += 1
+        else:
+            return False
+    return True
+
+
+assert orders_are_sorted([1,3,5], [2,4,6], [1,2,4,3,6,5])
+assert not orders_are_sorted([1,3,5], [2,4,6], [1,2,4,5,6,3])
+assert orders_are_sorted([], [2,4,6], [2,4,6])
+print("Success!")
diff --git a/users/wpcarro/scratch/facebook/cake_thief.py b/users/wpcarro/scratch/facebook/cake_thief.py
new file mode 100644
index 0000000000..90a2add066
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/cake_thief.py
@@ -0,0 +1,61 @@
+from math import floor
+
+def print_table(table):
+    print('\n-- TABLE --')
+    for row in range(len(table)):
+        x = ''
+        for col in range(len(table[row])):
+            x += ' ' + str(table[row][col])
+        print(x)
+
+def leftover(capacity, kg):
+    n = floor(capacity / kg)
+    return n, capacity - (n * kg)
+
+def init_table(num_rows, num_cols):
+    table = []
+    for _ in range(num_rows):
+        row = []
+        for _ in range(num_cols):
+            row.append(0)
+        table.append(row)
+    return table
+
+def get(table, row, col):
+    if row < 0 or col < 0:
+        return 0
+    return table[row][col]
+
+def max_haul(items, capacity):
+    table = init_table(len(items), capacity)
+
+    for row in range(len(table)):
+        for col in range(len(table[row])):
+            curr_capacity = col + 1
+            kg, val = items[row]
+            # A
+            a = get(table, row - 1, col)
+            # B
+            n, lo = leftover(curr_capacity, kg)
+            b = (val * n) + get(table, row - 1, lo - 1)
+            # commit
+            if kg > curr_capacity:
+                table[row][col] = a
+            else:
+                print(n, lo)
+                table[row][col] = max([a, b])
+            print_table(table)
+    return table[-1][-1]
+
+# There are multiple variants of this problem:
+#   1. We're allowed to take multiple of each item.
+#   2. We can only take one of each item.
+#   3. We can only take a fixed amount of each item.
+
+items = [(7,160), (3,90), (2,15)]
+capacity = 20
+result = max_haul(items, capacity)
+expected = None
+print("Result: {} == Expected: {}".format(result, expected))
+assert result == expected
+print("Success!")
diff --git a/users/wpcarro/scratch/facebook/camping-knapsack.py b/users/wpcarro/scratch/facebook/camping-knapsack.py
new file mode 100644
index 0000000000..add59ed409
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/camping-knapsack.py
@@ -0,0 +1,46 @@
+from utils import get, init_table, print_table
+
+def max_haul(capacity, items, names):
+    table = init_table(rows=len(items), cols=capacity, default=0)
+    items_table = init_table(rows=len(items), cols=capacity, default=[])
+    for row in range(len(table)):
+        for col in range(len(table[row])):
+            kg, value = items[row]
+            curr_capacity = col + 1
+
+            if kg > curr_capacity:
+                a = 0
+            else:
+                a = value + get(table, row - 1, curr_capacity - kg - 1)
+            b = get(table, row - 1, col)
+
+            if a > b:
+                rest = get(items_table, row - 1, curr_capacity - kg - 1)
+                knapsack = [names.get(items[row])]
+                if rest:
+                    knapsack += rest
+            else:
+                knapsack = get(items_table, row - 1, col)
+
+            table[row][col] = max([a, b])
+            items_table[row][col] = knapsack
+        print_table(table)
+    return items_table[-1][-1]
+
+water = (3, 10)
+book = (1, 3)
+food = (2, 9)
+jacket = (2, 5)
+camera = (1, 6)
+items = [water, book, food, jacket, camera]
+result = max_haul(6, items, {
+    water: 'water',
+    book: 'book',
+    food: 'food',
+    jacket: 'jacket',
+    camera: 'camera',
+})
+expected = ['camera', 'food', 'water']
+print(result, expected)
+assert result == expected
+print("Success!")
diff --git a/users/wpcarro/scratch/facebook/coin.py b/users/wpcarro/scratch/facebook/coin.py
new file mode 100644
index 0000000000..354e2dfb58
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/coin.py
@@ -0,0 +1,50 @@
+def init_table(rows=0, cols=0, default=None):
+    table = []
+    for _ in range(rows):
+        row = []
+        for _ in range(cols):
+            row.append(default)
+        table.append(row)
+    return table
+
+def print_table(table):
+    result = ''
+    for row in range(len(table)):
+        x = ''
+        for col in range(len(table[row])):
+            x += str(table[row][col]) + ' '
+        result += x + '\n'
+    print(result)
+
+def get(table, row, col):
+    if row < 0 or col < 0:
+        return 0
+    else:
+        return table[row][col]
+
+def make_change(coins, amt):
+    table = init_table(rows=len(coins), cols=amt, default=0)
+    for row in range(len(table)):
+        for col in range(len(table[row])):
+            coin = coins[row]
+            curr_amt = col + 1
+            pull_down = get(table, row - 1, col)
+
+            if curr_amt < coin:
+                table[row][col] = pull_down
+            elif curr_amt == coin:
+                table[row][col] = pull_down + 1
+            else:
+                leftover = get(table, row, curr_amt - coin - 1)
+                table[row][col] = pull_down + leftover
+
+    print_table(table)
+    return table[-1][-1]
+
+#   1 2 3 4
+# 1 1 1 1 1
+# 2 1 1 2 2
+# 3 1 1 3 4
+
+result = make_change([3,2,1], 4)
+print(result)
diff --git a/users/wpcarro/scratch/facebook/count-islands.py b/users/wpcarro/scratch/facebook/count-islands.py
new file mode 100644
index 0000000000..b876319b2f
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/count-islands.py
@@ -0,0 +1,53 @@
+from collections import deque
+
+def maybe_queue(row, col, game, q, seen):
+    """
+    Add coordinate, (`row`, `col`), to the queue, `q`, as long as it exists in
+    the map, `game`, and it is not already present in `seen`.
+    """
+    if row >= 0 and row < len(game) and col >= 0 and col < len(game[0]):
+        if game[row][col] == 'L' and (row, col) not in seen:
+            q.append((row, col))
+            seen.add((row, col))
+
+def visit_island(row, col, game, seen):
+    """
+    Starting at the coordinate, (`row`, `col`), in the map, `game`, visit all
+    surrounding tiles marked as land by adding them to the `seen` set.
+    """
+    q = deque()
+    q.append((row, col))
+    while q:
+        row, col = q.popleft()
+        maybe_queue(row - 1, col, game, q, seen) # UP
+        maybe_queue(row + 1, col, game, q, seen) # DOWN
+        maybe_queue(row, col - 1, game, q, seen) # LEFT
+        maybe_queue(row, col + 1, game, q, seen) # RIGHT
+
+def count_islands(game):
+    """
+    Return the number of contiguous land tiles in the map, `game`.
+    """
+    result = 0
+    seen = set()
+    for row in range(len(game)):
+        for col in range(len(game[row])):
+            if game[row][col] == 'L' and (row, col) not in seen:
+                visit_island(row, col, game, seen)
+                result += 1
+    return result
+
+################################################################################
+# Tests
+################################################################################
+
+game = [
+    "LWLWWW",
+    "LLLWWW",
+    "WWWLLW",
+]
+
+result = count_islands(game)
+print(result)
+assert result == 2
+print("Success!")
diff --git a/users/wpcarro/scratch/facebook/delete-node.py b/users/wpcarro/scratch/facebook/delete-node.py
new file mode 100644
index 0000000000..4034449ef0
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/delete-node.py
@@ -0,0 +1,19 @@
+from linked_list import Node, from_list
+
+def delete(node):
+    if not node.next:
+        node.value = None
+    else:
+        node.value = node.next.value
+        node.next = node.next.next
+
+one = Node(1)
+two = Node(2)
+three = Node(3)
+
+one.next = two
+two.next = three
+
+print(one)
+delete(two)
+print(one)
diff --git a/users/wpcarro/scratch/facebook/dijkstras.py b/users/wpcarro/scratch/facebook/dijkstras.py
new file mode 100644
index 0000000000..7031701994
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/dijkstras.py
@@ -0,0 +1,38 @@
+from heapq import heappush, heappop
+import random
+
+# Dijkstra's algorithm will traverse a directed graph with weighted edges. If
+# the edges aren't weighted, we can pretend that each edges weighs 1. The
+# algorithm will find the shortest path between points A and B.
+
+def dijkstra(a, b, graph):
+    h = []
+    seen = set()
+    heappush(h, (0, a, [a], []))
+    while h:
+        km, x, path, steps = heappop(h)
+
+        if x == b:
+            for a, b, d in steps:
+                print("{} -> {} => {}".format(a, b, d))
+            return path, km
+
+        seen.add(x)
+        for c, dist in graph[x]:
+            if c not in seen:
+                heappush(h, (km + dist, c, path + [c], steps + [(x, c, dist)]))
+    return [], float('inf')
+
+graph = {
+    1: [(3, 9), (2, 7), (6, 14)],
+    2: [(1, 7), (3, 10), (4, 15)],
+    3: [(1, 9), (6, 2), (4, 11), (2, 10)],
+    4: [(5, 6), (2, 15), (3, 11)],
+    5: [(4, 6), (6, 9)],
+    6: [(5, 9), (3, 2), (1, 14)],
+}
+
+beg = random.choice(list(graph.keys()))
+end = random.choice(list(graph.keys()))
+print("Searching for the shortest path from {} -> {}".format(beg, end))
+print(dijkstra(beg, end, graph))
diff --git a/users/wpcarro/scratch/facebook/edit-distance.py b/users/wpcarro/scratch/facebook/edit-distance.py
new file mode 100644
index 0000000000..a5b744f30f
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/edit-distance.py
@@ -0,0 +1,47 @@
+def print_grid(grid):
+    result = []
+    for row in grid:
+        result.append(" ".join(str(c) for c in row))
+    return print("\n".join(result))
+
+def edit_distance(a, b):
+    """
+    Compute the "edit distance" to transform string `a` into string `b`.
+    """
+    grid = []
+    for row in range(len(a) + 1):
+        r = []
+        for col in range(len(b) + 1):
+            r.append(0)
+        grid.append(r)
+
+    # left-to-right
+    # populate grid[0][i]
+    for col in range(len(grid[0])):
+        grid[0][col] = col
+
+    # top-to-bottom
+    # populate grid[i][0]
+    for row in range(len(grid)):
+        grid[row][0] = row
+
+    for row in range(1, len(grid)):
+        for col in range(1, len(grid[row])):
+            # last characters are the same
+            if a[0:row][-1] == b[0:col][-1]:
+                grid[row][col] = grid[row - 1][col - 1]
+            else:
+                # substitution
+                s = 1 + grid[row - 1][col - 1]
+                # deletion
+                d = 1 + grid[row - 1][col]
+                # insertion
+                i = 1 + grid[row][col - 1]
+                grid[row][col] = min(s, d, i)
+    print_grid(grid)
+    return grid[-1][-1]
+
+result = edit_distance("pizza", "pisa")
+print(result)
+assert result == 2
+print("Success!")
diff --git a/users/wpcarro/scratch/facebook/evaluator.hs b/users/wpcarro/scratch/facebook/evaluator.hs
new file mode 100644
index 0000000000..1ba46a7548
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/evaluator.hs
@@ -0,0 +1,39 @@
+module Evaluator where
+
+data Token
+  = TokenInt Integer
+  | TokenAdd
+  | TokenMultiply
+  deriving (Eq, Show)
+
+newtype AST = AST [Token]
+  deriving (Eq, Show)
+
+tokens :: [Token]
+tokens =
+  [ TokenInt 13
+  , TokenAdd
+  , TokenInt 2
+  , TokenMultiply
+  , TokenInt 4
+  , TokenAdd
+  , TokenInt 7
+  , TokenAdd
+  , TokenInt 3
+  , TokenMultiply
+  , TokenInt 8
+  ]
+
+-- expression     -> addition ;
+-- addition       -> multiplication ( "+" multiplication )* ;
+-- multiplication -> terminal ( "*" terminal )* ;
+-- terminal       -> NUMBER ;
+
+parseExpression :: [Token] -> ([Token], AST)
+parseExpression tokens = do
+  lhs, rest = parseMultiplication tokens
+
+parseMulitplication :: [Token] -> ([Token], AST)
+
+main :: IO ()
+main = print $ parse tokens
diff --git a/users/wpcarro/scratch/facebook/evaluator.py b/users/wpcarro/scratch/facebook/evaluator.py
new file mode 100644
index 0000000000..14deb66a8f
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/evaluator.py
@@ -0,0 +1,234 @@
+# After stumbling through my first technical screen, I'm going to drill
+# algorithms for implementing evaluators for a toy expression language:
+# e.g. 2 + 13 * 3 + 5 * 2
+#
+# As of now, I'm aware of a few algorithms for solving this:
+#   - DONE: Convert infix expression to Polish notation and evaluate the Polish
+#     notation.
+#   - DONE: Evaluate the tokens using two stacks and avoid converting it.
+#   - DONE: Create a tree of depth two to encode the operator precedence and
+#     evaluate that AST.
+#   - TODO: Convert the infix expression to a prefix expression
+#   - TODO: Write a recursive descent parser and evaluate the AST.
+
+operators = {
+    '*': 1,
+    '+': 0,
+}
+
+def tokenize(xs):
+    result = []
+    i = 0
+    while i < len(xs):
+        current = xs[i]
+        if current == ' ':
+            i += 1
+            continue
+        elif current in operators.keys():
+            result.append(current)
+            i += 1
+        else:
+            i += 1
+            while i < len(xs) and xs[i] in {str(n) for n in range(10)}:
+                current += xs[i]
+                i += 1
+            result.append(int(current))
+    return result
+
+# Convert infix to postfix; evaluate postfix
+# I believe this is known as the Shunting-Yards algorithm
+def postfix(tokens):
+    result = []
+    s = []
+    for token in tokens:
+        if type(token) == int:
+            result.append(token)
+        else:
+            while s and operators[token] < operators[s[-1]]:
+                result.append(s.pop())
+            s.append(token)
+    while s:
+        result.append(s.pop())
+    return result
+
+def do_evaluate_with_polish_notation(tokens):
+    s = []
+    for token in tokens:
+        if token == '*':
+            s.append(s.pop() * s.pop())
+        elif token == '+':
+            s.append(s.pop() + s.pop())
+        else:
+            s.append(token)
+    return s[-1]
+
+def evaluate_with_polish_notation(expr):
+    tokens = tokenize(expr)
+    print("Tokens:  {}".format(tokens))
+    pn = postfix(tokens)
+    print("Postfix: {}".format(pn))
+    result = do_evaluate_with_polish_notation(pn)
+    print("Result:  {}".format(result))
+    return result
+
+# Evaluate Tokens
+
+def apply_operator(op, a, b):
+    if op == '*':
+        return a * b
+    elif op == '+':
+        return a + b
+
+def do_evaluate_tokens(tokens):
+    vals = []
+    ops = []
+    for token in tokens:
+        if type(token) == int:
+            vals.append(token)
+        elif token == '*':
+            ops.append(token)
+        elif token == '+':
+            while ops and operators[token] < operators[ops[-1]]:
+                vals.append(apply_operator(ops.pop(), vals.pop(), vals.pop()))
+            ops.append(token)
+        else:
+            raise Exception("Unexpected token: {}".format(token))
+    while ops:
+        vals.append(apply_operator(ops.pop(), vals.pop(), vals.pop()))
+    return vals[-1]
+
+def evaluate_tokens(expr):
+    tokens = tokenize(expr)
+    print("Tokens:  {}".format(tokens))
+    result = do_evaluate_tokens(tokens)
+    print("Result:  {}".format(result))
+    return result
+
+# Ad Hoc Tree
+
+def parse(tokens):
+    result = []
+    series = []
+    for token in tokens:
+        if type(token) == int:
+            series.append(token)
+        elif token == '*':
+            continue
+        elif token == '+':
+            result.append(series)
+            series = []
+        else:
+            raise Exception("Unexpected token: {}".format(token))
+    result.append(series)
+    return result
+
+def product(xs):
+    result = 1
+    for x in xs:
+        result *= x
+    return result
+
+def do_evaluate_ad_hoc_tree(ast):
+    return sum([product(xs) for xs in ast])
+
+def evaluate_ad_hoc_tree(expr):
+    tokens = tokenize(expr)
+    print("Tokens:  {}".format(tokens))
+    ast = parse(tokens)
+    print("AST:     {}".format(ast))
+    result = do_evaluate_ad_hoc_tree(ast)
+    print("Result:  {}".format(result))
+    return result
+
+# Recursive Descent Parser
+
+# expression     -> addition ;
+# addition       -> multiplication ( "+" multiplication )* ;
+# multiplication -> terminal ( "*" terminal )* ;
+# terminal       -> NUMBER ;
+
+class Parser(object):
+    def __init__(self, tokens):
+        self.tokens = tokens
+        self.i = 0
+
+    # mutations
+    def advance(self):
+        self.i += 1
+
+    def consume(self):
+        result = self.curr()
+        self.advance()
+        return result
+
+    # predicates
+    def match(self, x):
+        if self.curr() == x:
+            self.advance()
+            return True
+        return False
+
+    def tokens_available(self):
+        return self.i < len(self.tokens)
+
+    # getters
+    def prev(self):
+        return self.tokens[self.i - 1]
+
+    def curr(self):
+        return self.tokens[self.i] if self.tokens_available() else None
+
+    def next(self):
+        return self.tokens[self.i + 1]
+
+def parse_expression(tokens):
+    parser = Parser(tokens)
+    return parse_addition(parser)
+
+def parse_addition(parser):
+    result = parse_multiplication(parser)
+    while parser.match("+"):
+        op = parser.prev()
+        rhs = parse_multiplication(parser)
+        result = ["+", result, rhs]
+    return result
+
+def parse_multiplication(parser):
+    result = parse_terminal(parser)
+    while parser.match("*"):
+        op = parser.prev()
+        rhs = parse_terminal(parser)
+        result = ["*", result, rhs]
+    return result
+
+def parse_terminal(parser):
+    # If we reach here, the current token *must* be a number.
+    return parser.consume()
+
+def evaluate_ast(ast):
+    if type(ast) == int:
+        return ast
+    else:
+        op, lhs, rhs = ast[0], ast[1], ast[2]
+        return apply_operator(op, evaluate_ast(lhs), evaluate_ast(rhs))
+
+def evaluate_recursive_descent(expr):
+    tokens = tokenize(expr)
+    print("Tokens:  {}".format(tokens))
+    ast = parse_expression(tokens)
+    print("AST:     {}".format(ast))
+    result = evaluate_ast(ast)
+    return result
+
+methods = {
+    'Polish Notation': evaluate_with_polish_notation,
+    'Evaluate Tokens': evaluate_tokens,
+    'Ad Hoc Tree': evaluate_ad_hoc_tree,
+    'Recursive Descent': evaluate_recursive_descent,
+}
+
+for name, fn in methods.items():
+    expr = "13 + 2 * 4 + 7 + 3 * 8"
+    print("Evaluating \"{}\" using the \"{}\" method...".format(expr, name))
+    assert fn(expr) == eval(expr)
+    print("Success!")
diff --git a/users/wpcarro/scratch/facebook/find-duplicate-beast-mode.py b/users/wpcarro/scratch/facebook/find-duplicate-beast-mode.py
new file mode 100644
index 0000000000..e246415efd
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/find-duplicate-beast-mode.py
@@ -0,0 +1,57 @@
+def advance(position, xs):
+    """
+    Return the next element in `xs` pointed to by the current `position`.
+    """
+    return xs[position - 1]
+
+def find_duplicate(xs):
+    """
+    Find the duplicate integer in the list, `xs`.
+    """
+    beg = xs[-1]
+    a = beg
+    b = advance(a, xs)
+    # Find the first element of the cycle
+    cycle_beg = None
+    while a != b:
+        cycle_beg = a
+        a = advance(a, xs)
+        b = advance(b, xs)
+        b = advance(b, xs)
+    # The duplicate element is the element before the `cycle_beg`
+    a = beg
+    result = None
+    while a != cycle_beg:
+        result = a
+        a = advance(a, xs)
+    return result
+
+def find_duplicate(xs):
+    """
+    This is the solution that InterviewCake.com suggests.
+    """
+    # find length of the cycle
+    beg = xs[-1]
+    a = beg
+    for _ in range(len(xs)):
+        a = advance(a, xs)
+    element = a
+    a = advance(a, xs)
+    n = 1
+    while a != element:
+        a = advance(a, xs)
+        n += 1
+    # find the first element in the cycle
+    a, b = beg, beg
+    for _ in range(n):
+        b = advance(b, xs)
+    while a != b:
+        a = advance(a, xs)
+        b = advance(b, xs)
+    return a
+
+xs = [2, 3, 1, 3]
+result = find_duplicate(xs)
+print(result)
+assert result == 3
+print("Success!")
diff --git a/users/wpcarro/scratch/facebook/find-duplicate-optimize-for-space.py b/users/wpcarro/scratch/facebook/find-duplicate-optimize-for-space.py
new file mode 100644
index 0000000000..7c491aef60
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/find-duplicate-optimize-for-space.py
@@ -0,0 +1,22 @@
+import random
+
+def find_duplicate(xs):
+    print(xs)
+    # entry point in our cycle is the duplicate
+    i = xs[0]
+    j = xs[xs[0]]
+    while i != j:
+        print(i, xs[i], j, xs[j])
+        i = xs[i]
+        j = xs[xs[j]]
+    # detect cycle
+    j = 0
+    while i != j:
+        i = xs[i]
+        j = xs[j]
+    return xs[i]
+
+n = random.randint(5, 10)
+xs = [random.randint(0, n - 1) for _ in range(n)]
+result = find_duplicate(xs)
+print(xs, result)
diff --git a/users/wpcarro/scratch/facebook/find-rotation-point.py b/users/wpcarro/scratch/facebook/find-rotation-point.py
new file mode 100644
index 0000000000..3636be4d93
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/find-rotation-point.py
@@ -0,0 +1,47 @@
+from math import floor
+
+def find_rotation(xs):
+    if xs[0] < xs[-1]:
+        return xs[0]
+    beg, end = 0, len(xs) - 1
+    found = False
+    count = 10
+    while not found and count >= 0:
+        i = beg + floor((end - beg) / 2)
+        if xs[beg] < xs[i]:
+            beg = i
+            i = beg + floor((end - beg) / 2)
+        elif xs[beg] > xs[i]:
+            end = i
+        found = xs[i - 1] > xs[i]
+        count -= 1
+    return xs[i]
+
+
+xs = [(['ptolemaic',
+        'retrograde',
+        'supplant',
+        'undulate',
+        'xenoepist',
+        'zebra',
+        'asymptote',
+        'babka',
+        'banoffee',
+        'engender',
+        'karpatka',
+        'othellolagkage',
+        ], "asymptote"),
+      (['asymptote',
+        'babka',
+        'banoffee',
+        'engender',
+        'karpatka',
+        'othellolagkage',
+        ], "asymptote"),
+      ]
+
+for x, expected in xs:
+    result = find_rotation(x)
+    print(x, result)
+    assert result == expected
+    print("Success!")
diff --git a/users/wpcarro/scratch/facebook/find-unique-int-among-duplicates.py b/users/wpcarro/scratch/facebook/find-unique-int-among-duplicates.py
new file mode 100644
index 0000000000..56032aa05c
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/find-unique-int-among-duplicates.py
@@ -0,0 +1,17 @@
+import random
+
+def find_duplicate(xs):
+    mini, maxi, acc = xs[0], xs[0], xs[0]
+    for i in range(1, len(xs)):
+        mini = min(mini, xs[i])
+        maxi = max(maxi, xs[i])
+        acc = acc ^ xs[i]
+    mask = mini
+    for i in range(mini + 1, maxi + 1):
+        mask = mask ^ i
+    return mask ^ acc
+
+xs = [5, 3, 4, 1, 5, 2]
+print(xs)
+result = find_duplicate(xs)
+print(result)
diff --git a/users/wpcarro/scratch/facebook/graph-coloring.py b/users/wpcarro/scratch/facebook/graph-coloring.py
new file mode 100644
index 0000000000..e5b6d9c893
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/graph-coloring.py
@@ -0,0 +1,60 @@
+from collections import deque
+
+class Palette(object):
+    def __init__(self, n):
+        self.i = 0
+        self.colors = list(range(n))
+
+    def get(self):
+        return self.colors[self.i]
+
+    def advance(self):
+        self.i += 1 % len(self.colors)
+
+class GraphNode(object):
+    def __init__(self, label):
+        self.label = label
+        self.neighbors = set()
+        self.color = None
+
+    def __repr__(self):
+        result = []
+        xs = deque()
+        xs.append(self)
+        seen = set()
+        while xs:
+            node = xs.popleft()
+            result.append('{} ({})'.format(node.label, str(node.color)))
+            for c in node.neighbors:
+                if c.label not in seen:
+                    xs.append(c)
+                    seen.add(node.label)
+        return ', '.join(result)
+
+def color_graph(graph, d):
+    seen = set()
+    start = graph
+    xs = deque()
+    palette = Palette(d + 1)
+    xs.append((start, palette.get()))
+    while xs:
+        x, color = xs.popleft()
+        x.color = color
+        for c in x.neighbors:
+            if c.label not in seen:
+                palette.advance()
+                xs.append((c, palette.get()))
+                seen.add(x.label)
+
+a = GraphNode('a')
+b = GraphNode('b')
+c = GraphNode('c')
+
+a.neighbors.add(b)
+b.neighbors.add(a)
+b.neighbors.add(c)
+c.neighbors.add(b)
+
+print(a)
+color_graph(a, 3)
+print(a)
diff --git a/users/wpcarro/scratch/facebook/hard/binary-adder.py b/users/wpcarro/scratch/facebook/hard/binary-adder.py
new file mode 100644
index 0000000000..f79a9f22b3
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/hard/binary-adder.py
@@ -0,0 +1,22 @@
+import random
+
+def add(a, b):
+    """
+    Return the sum of `a` and `b`.
+    """
+    if b == 0:
+        return a
+    sum = a ^ b
+    carry = (a & b) << 1
+    return add(sum, carry)
+
+################################################################################
+# Tests
+################################################################################
+
+for _ in range(10):
+    x, y = random.randint(0, 100), random.randint(0, 100)
+    print("{} + {} = {} == {}".format(x, y, x + y, add(x, y)))
+    assert add(x, y) == x + y
+    print("Pass!")
+print("Success!")
diff --git a/users/wpcarro/scratch/facebook/hard/fisher-yates.py b/users/wpcarro/scratch/facebook/hard/fisher-yates.py
new file mode 100644
index 0000000000..200d1613dd
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/hard/fisher-yates.py
@@ -0,0 +1,7 @@
+import random
+
+def shuffle(xs):
+    n = len(xs)
+    for i in range(n):
+        j = random.randint(i, n - 1)
+        xs[i], xs[j] = xs[j], xs[i]
diff --git a/users/wpcarro/scratch/facebook/hard/random-choice.py b/users/wpcarro/scratch/facebook/hard/random-choice.py
new file mode 100644
index 0000000000..a5c6e4e6ee
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/hard/random-choice.py
@@ -0,0 +1,50 @@
+import random
+
+# This class of problems is known as "resevoir sampling".
+def choose_a(m, xs):
+    """
+    Randomly choose `m` elements from `xs`.
+    This algorithm runs in linear time with respect to the size of `xs`.
+    """
+    result = [None] * m
+    for i in range(len(xs)):
+        j = random.randint(0, i)
+        if j < m:
+            result[j] = xs[i]
+    return result
+
+def choose_b(m, xs):
+    """
+    This algorithm, which copies `xs`, which runs in linear time, and then
+    shuffles the copies, which also runs in linear time, achieves the same
+    result as `choose_a` and both run in linear time.
+
+    `choose_a` is still preferable since it has a coefficient of one, while this
+    version has a coefficient of two because it copies + shuffles.
+    """
+    ys = xs[:]
+    random.shuffle(ys)
+    return ys[:m]
+
+def choose_c(m, xs):
+    """
+    This is one, possibly inefficient, way to randomly sample `m` elements from
+    `xs`.
+    """
+    choices = set()
+    while len(choices) < m:
+        choices.add(random.randint(0, len(xs) - 1))
+    return [xs[i] for i in choices]
+
+# ROYGBIV
+xs = [
+    'red',
+    'orange',
+    'yellow',
+    'green',
+    'blue',
+    'indigo',
+    'violet',
+]
+print(choose_b(3, xs))
+print(choose_c(3, xs))
diff --git a/users/wpcarro/scratch/facebook/hard/suffix-tree.py b/users/wpcarro/scratch/facebook/hard/suffix-tree.py
new file mode 100644
index 0000000000..782678fb82
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/hard/suffix-tree.py
@@ -0,0 +1,93 @@
+import random
+from collections import deque
+
+def exists(pattern, tree):
+    """
+    Return true if `pattern` exists in `tree`.
+    """
+    if len(pattern) == 0:
+        return True
+    if len(pattern) == 1:
+        for branch in tree:
+            if branch[0] == pattern[0]:
+                return True
+        return False
+    for branch in tree:
+        if branch[0] == pattern[0]:
+            return exists(pattern[1:], branch[1])
+    return False
+
+# Branch :: (Char, [Branch])
+# SuffixTree :: [Branch]
+
+def suffix_tree(xs):
+    """
+    Create a suffix tree from the input string, `xs`.
+    """
+    root = []
+    for i in range(len(xs)):
+        curr = xs[i:]
+        parent = root
+        for c1 in curr:
+            grafted = False
+            for c2, children in parent:
+                if c1 == c2:
+                    grafted = True
+                    parent = children
+            if grafted:
+                continue
+            else:
+                children = []
+                child = (c1, children)
+                parent.append(child)
+                parent = children
+    return root
+
+def suffix_tree(x):
+    """
+    Creates a suffix from the input string, `x`. This implementation uses a
+    stack.
+    """
+    result = [None, []]
+    q = deque()
+    for i in range(len(x)):
+        q.append((result, x[i:]))
+    while q:
+        parent, x = q.popleft()
+        s = []
+        s.append((parent, x))
+        while s:
+            parent, x = s.pop()
+            if not x:
+                continue
+            c, rest = x[0], x[1:]
+            grafted = False
+            for child in parent[1]:
+                if c == child[0]:
+                    s.append((child, rest))
+                    grafted = True
+            if not grafted:
+                child = [c, []]
+                parent[1].append(child)
+                s.append((child, rest))
+    return result[1]
+
+################################################################################
+# Tests
+################################################################################
+
+x = random.choice(["burrito", "pizza", "guacamole"])
+tree = suffix_tree(x)
+for branch in tree:
+    print(branch)
+
+for _ in range(3):
+    n = len(x)
+    i, j = random.randint(0, n), random.randint(0, n)
+    pattern = x[min(i, j):max(i, j)]
+    print("Checking \"{}\" for \"{}\" ...".format(x, pattern))
+    print("Result: {}".format(exists(pattern, tree)))
+    pattern = random.choice(["foo", "bar", "baz"])
+    print("Checking \"{}\" for \"{}\" ...".format(x, pattern))
+    print("Result: {}".format(exists(pattern, tree)))
+    print()
diff --git a/users/wpcarro/scratch/facebook/heap.py b/users/wpcarro/scratch/facebook/heap.py
new file mode 100644
index 0000000000..0c0dce91b4
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/heap.py
@@ -0,0 +1,30 @@
+from math import floor
+
+class Heap(object):
+    def __init__(self):
+        self.xs = [None]
+        self.i = 1
+
+    def __repr__(self):
+        return "[{}]".format(", ".join(str(x) for x in self.xs[1:]))
+
+    def insert(self, x):
+        if len(self.xs) == 1:
+            self.xs.append(x)
+            self.i += 1
+            return
+        self.xs.append(x)
+        i = self.i
+        while i != 1 and self.xs[floor(i / 2)] > self.xs[i]:
+            self.xs[floor(i / 2)], self.xs[i] = self.xs[i], self.xs[floor(i / 2)]
+            i = floor(i / 2)
+        self.i += 1
+
+    def root(self):
+        return self.xs[1]
+
+xs = Heap()
+print(xs)
+for x in [12, 15, 14, 21, 1, 10]:
+    xs.insert(x)
+    print(xs)
diff --git a/users/wpcarro/scratch/facebook/highest-product-of-3.py b/users/wpcarro/scratch/facebook/highest-product-of-3.py
new file mode 100644
index 0000000000..c237b8e52e
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/highest-product-of-3.py
@@ -0,0 +1,20 @@
+def hi_product(xs):
+    lowest_one, highest_one = min(xs[0], xs[1]), max(xs[0], xs[1])
+    lowest_two, highest_two = xs[0] * xs[1], xs[0] * xs[1]
+    highest = float('-inf')
+    for x in xs[2:]:
+        highest = max(highest, highest_two * x, lowest_two * x)
+        lowest_one = min(lowest_one, x)
+        highest_one = max(highest_one, x)
+        lowest_two = min(lowest_two, highest_one * x, lowest_one * x)
+        highest_two = max(highest_two, highest_one * x, lowest_one * x)
+    return highest
+
+xs = [([-10,-10,1,3,2], 300),
+      ([1,10,-5,1,-100], 5000)]
+
+for x, expected in xs:
+    result = hi_product(x)
+    print(x, result)
+    assert result == expected
+    print("Success!")
diff --git a/users/wpcarro/scratch/facebook/infix-to-postfix.py b/users/wpcarro/scratch/facebook/infix-to-postfix.py
new file mode 100644
index 0000000000..4c6d64494d
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/infix-to-postfix.py
@@ -0,0 +1,51 @@
+operators = {
+    '*': 1,
+    '+': 0,
+}
+
+def tokenize(xs):
+    result = []
+    i = 0
+    while i < len(xs):
+        current = xs[i]
+        if current in operators.keys():
+            result.append(current)
+            i += 1
+            continue
+        else:
+            i += 1
+            while i < len(xs) and xs[i] in {str(n) for n in range(10)}:
+                current += xs[i]
+                i += 1
+            result.append(int(current))
+    return result
+
+def postfix(xs):
+    result = []
+    s = []
+    for x in xs:
+        if x in operators.keys():
+            while s and operators[s[-1]] >= operators[x]:
+                result.append(s.pop())
+            s.append(x)
+        else:
+            result.append(x)
+    while s:
+        result.append(s.pop())
+    return result
+
+def evaluate(xs):
+    s = []
+    for x in xs:
+        print(s, x)
+        if x == '*':
+            s.append(s.pop() * s.pop())
+        elif x == '+':
+            s.append(s.pop() + s.pop())
+        else:
+            s.append(x)
+        print(s)
+    return s[-1]
+
+
+print(evaluate(postfix(tokenize("12+3*10"))))
diff --git a/users/wpcarro/scratch/facebook/inflight-entertainment.py b/users/wpcarro/scratch/facebook/inflight-entertainment.py
new file mode 100644
index 0000000000..7ddea5350a
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/inflight-entertainment.py
@@ -0,0 +1,29 @@
+from random import choice
+from utils import init_table
+
+def get(movie, seeking):
+    return any([movie in xs for xs in seeking.values()])
+
+def set_complement(movie, seeking):
+    for duration, xs in seeking.items():
+        seeking[duration].add(duration - movie)
+
+def choose_movies(tolerance, duration, movies):
+    seeking = {duration + i: set() for i in range(-1 * tolerance, tolerance + 1)}
+    for movie in movies:
+        if get(movie, seeking):
+            return movie, duration - movie
+        else:
+            set_complement(movie, seeking)
+    return None
+
+tolerance = 20
+duration = choice([1, 2, 3]) * choice([1, 2]) * choice([15, 30, 45])
+movies = [choice([1, 2, 3]) * choice([15, 30, 45]) for _ in range(10)]
+print("Seeking two movies for a duration of [{}, {}] minutes".format(duration - tolerance, duration + tolerance))
+print(movies)
+result = choose_movies(tolerance, duration, movies)
+if result:
+    print("{} + {} = {}".format(result[0], result[1], duration))
+else:
+    print(":( We're sad because we couldn't find two movies for a {} minute flight".format(duration))
diff --git a/users/wpcarro/scratch/facebook/intersecting-linked-lists.py b/users/wpcarro/scratch/facebook/intersecting-linked-lists.py
new file mode 100644
index 0000000000..80ac01dafd
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/intersecting-linked-lists.py
@@ -0,0 +1,34 @@
+class LinkedList(object):
+    def __init__(self, x):
+        self.val = x
+        self.next = None
+
+    def __repr__(self):
+        if self.next:
+            return "{} -> {}".format(self.val, self.next)
+        return "{}".format(self.val)
+
+def find_intersection(a, b):
+    init_a, init_b = a, b
+
+    while a != b:
+        a = a.next if a.next else init_b
+        b = b.next if b.next else init_a
+
+    return a
+
+# make A...
+e1 = LinkedList(5)
+d1 = LinkedList(2); d1.next = e1
+c1 = LinkedList(3); c1.next = d1 # shared
+b1 = LinkedList(1); b1.next = c1 # shared
+a1 = LinkedList(4); a1.next = b1 # shared
+
+# make B...
+c2 = LinkedList(1); c2.next = c1
+b2 = LinkedList(5); b2.next = c2
+a2 = LinkedList(6); a2.next = b2
+
+print(a1)
+print(a2)
+print(find_intersection(a1, a2).val)
diff --git a/users/wpcarro/scratch/facebook/interview-cake/bst-checker.py b/users/wpcarro/scratch/facebook/interview-cake/bst-checker.py
new file mode 100644
index 0000000000..bbd52fa9c6
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/interview-cake/bst-checker.py
@@ -0,0 +1,14 @@
+def is_valid(node):
+    """
+    Return True if `node` is a valid binary search tree.
+    """
+    s = []
+    s.append((float('-inf'), node, float('inf')))
+    while s:
+        lo, node, hi = s.pop()
+        if lo <= node.value <= hi:
+            node.lhs and s.append((lo, node.lhs, node.value))
+            node.rhs and s.append((node.value, node.rhs, hi))
+        else:
+            return False
+    return True
diff --git a/users/wpcarro/scratch/facebook/interview-cake/cafe-order-checker.py b/users/wpcarro/scratch/facebook/interview-cake/cafe-order-checker.py
new file mode 100644
index 0000000000..688c340b98
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/interview-cake/cafe-order-checker.py
@@ -0,0 +1,34 @@
+def valid(take_out, dine_in, served):
+    # edge case
+    if len(take_out) + len(dine_in) != len(served):
+        return False
+    i = 0
+    j = 0
+    k = 0
+    while i < len(take_out) and j < len(dine_in):
+        if take_out[i] == served[k]:
+            i += 1
+        elif dine_in[j] == served[k]:
+            j += 1
+        else:
+            return False
+        k += 1
+    # take out
+    while i < len(take_out):
+        if take_out[i] != served[k]:
+            return False
+        i += 1
+    # dine in
+    while j < len(dine_in):
+        if dine_in[j] != served[k]:
+            return False
+        j += 1
+    return True
+
+take_out = [17, 8, 24]
+dine_in  = [12, 19, 2]
+served   = [17, 8, 12, 19, 24, 2]
+result = valid(take_out, dine_in, served)
+print(result)
+assert result
+print("Success!")
diff --git a/users/wpcarro/scratch/facebook/interview-cake/linked-list-cycles.py b/users/wpcarro/scratch/facebook/interview-cake/linked-list-cycles.py
new file mode 100644
index 0000000000..523ecd959d
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/interview-cake/linked-list-cycles.py
@@ -0,0 +1,70 @@
+def contains_cycle(node):
+    """
+    Return True if the linked-list, `node`, contains a cycle.
+    """
+    if not node:
+        return False
+    a = node
+    b = node.next
+    while a != b:
+        a = a.next
+        if b and b.next and b.next.next:
+            b = b.next.next
+        else:
+            return False
+    return True
+
+################################################################################
+# Bonus
+################################################################################
+
+def first_node_in_cycle(node):
+    """
+    Given that the linked-list, `node`, contains a cycle, return the first
+    element of that cycle.
+    """
+    # enter the cycle
+    a = node
+    b = node.next
+    while a != b:
+        a = a.next
+        b = b.next.next
+
+    # get the length of the cycle
+    beg = a
+    a = a.next
+    n = 1
+    while a != beg:
+        a = a.next
+        n += 1
+
+    # run b n-steps ahead of a
+    a = node
+    b = node
+    for _ in range(n):
+        b = b.next
+
+    # where they intersect is the answer
+    while a != b:
+        a = a.next
+        b = b.next
+    return a
+
+################################################################################
+# Tests
+################################################################################
+
+class Node(object):
+    def __init__(self, value, next=None):
+        self.value = value
+        self.next = next
+    def __repr__(self):
+        return "Node({}) -> ...".format(self.value)
+
+d = Node('d')
+c = Node('c', d)
+b = Node('b', c)
+a = Node('a', b)
+d.next = b
+
+print(first_node_in_cycle(a))
diff --git a/users/wpcarro/scratch/facebook/interview-cake/merge-sorted-arrays.py b/users/wpcarro/scratch/facebook/interview-cake/merge-sorted-arrays.py
new file mode 100644
index 0000000000..877bb218fd
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/interview-cake/merge-sorted-arrays.py
@@ -0,0 +1,30 @@
+def merge_sorted(xs, ys):
+    result = []
+    i = 0
+    j = 0
+    while i < len(xs) and j < len(ys):
+        if xs[i] <= ys[j]:
+            result.append(xs[i])
+            i += 1
+        else:
+            result.append(ys[j])
+            j += 1
+    while i < len(xs):
+        result.append(xs[i])
+        i += 1
+    while j < len(xs):
+        result.append(ys[j])
+        j += 1
+    return result
+
+################################################################################
+# Tests
+################################################################################
+
+xs = [3, 4, 6, 10, 11, 15]
+ys = [1, 5, 8, 12, 14, 19]
+result = merge_sorted(xs, ys)
+print(result)
+assert len(result) == len(xs) + len(ys)
+assert result == [1, 3, 4, 5, 6, 8, 10, 11, 12, 14, 15, 19]
+print("Success!")
diff --git a/users/wpcarro/scratch/facebook/interview-cake/nth-fibonacci.py b/users/wpcarro/scratch/facebook/interview-cake/nth-fibonacci.py
new file mode 100644
index 0000000000..4629798cf7
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/interview-cake/nth-fibonacci.py
@@ -0,0 +1,6 @@
+def fib(n):
+    cache = (0, 1)
+    for _ in range(n):
+        a, b = cache
+        cache = (b, a + b)
+    return cache[0]
diff --git a/users/wpcarro/scratch/facebook/interview-cake/permutation-palindrome.py b/users/wpcarro/scratch/facebook/interview-cake/permutation-palindrome.py
new file mode 100644
index 0000000000..ced3b336e0
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/interview-cake/permutation-palindrome.py
@@ -0,0 +1,8 @@
+from collections import Counter
+
+def permutation_can_be_palindrome(x):
+    odd = 0
+    for _, n in Counter(x):
+        if n % 0 != 0:
+            odd += 1
+    return odd <= 1
diff --git a/users/wpcarro/scratch/facebook/interview-cake/queue-two-stacks.py b/users/wpcarro/scratch/facebook/interview-cake/queue-two-stacks.py
new file mode 100644
index 0000000000..bfa465f98d
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/interview-cake/queue-two-stacks.py
@@ -0,0 +1,17 @@
+class Queue(object):
+    def __init__(self):
+        self.lhs = []
+        self.rhs = []
+
+    def enqueue(self, x):
+        self.lhs.append(x)
+
+    def dequeue(self):
+        if self.rhs:
+            return self.rhs.pop()
+        while self.lhs:
+            self.rhs.append(self.lhs.pop())
+        if self.rhs:
+            return self.rhs.pop()
+        else:
+            raise Exception("Attempting to remove an item from an empty queue")
diff --git a/users/wpcarro/scratch/facebook/knapsack-faq.py b/users/wpcarro/scratch/facebook/knapsack-faq.py
new file mode 100644
index 0000000000..ae04f5eb96
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/knapsack-faq.py
@@ -0,0 +1,42 @@
+from utils import get, init_table, print_table
+
+# This problem has a few variants:
+#   - limited supply of each item
+#   - unlimited supply of each item
+#   - fractional amounts of each item (e.g. rice)
+
+def max_haul(capacity, items):
+    min_kg = min([kg for _, kg in items])
+    max_kg = max([kg for _, kg in items])
+
+    cols = int(max_kg / min_kg)
+    fr_col_index = lambda index: min_kg * index + min_kg
+    to_col_index = lambda capacity: int((capacity - min_kg) * cols / max_kg)
+
+    table = init_table(rows=len(items), cols=cols, default=0)
+    for row in range(len(table)):
+        for col in range(len(table[row])):
+            curr_capacity = fr_col_index(col)
+            value, kg = items[row]
+
+            if kg > curr_capacity:
+                a = 0
+            else:
+                a = value + get(table, row - 1, to_col_index(curr_capacity - kg))
+
+            b = get(table, row - 1, col)
+            table[row][col] = max([a, b])
+        print_table(table)
+    return table[-1][-1]
+
+guitar = (1500, 1)
+stereo = (3000, 4)
+laptop = (2000, 3)
+necklace = (2000, 0.5)
+items = [necklace, guitar, stereo, laptop]
+capacity = 4
+result = max_haul(capacity, items)
+expected = 4000
+print(result, expected)
+assert result == expected
+print("Success!")
diff --git a/users/wpcarro/scratch/facebook/kth-to-last-node-in-singly-linked-list.py b/users/wpcarro/scratch/facebook/kth-to-last-node-in-singly-linked-list.py
new file mode 100644
index 0000000000..dd258d924d
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/kth-to-last-node-in-singly-linked-list.py
@@ -0,0 +1,26 @@
+from linked_list import Node, from_list
+
+def kth_to_last_node(k, node):
+    one = node
+    two = node
+    for _ in range(k - 1):
+        if not one:
+            return None
+        one = one.next
+    while one.next:
+        one = one.next
+        two = two.next
+    return two.value
+
+
+xs = from_list(["Angel Food", "Bundt", "Cheese", "Devil's Food", "Eccles"])
+result = kth_to_last_node(2, xs)
+print(result)
+assert result == "Devil's Food"
+print("Success!")
+
+xs = from_list(["Angel Food", "Bundt"])
+result = kth_to_last_node(30, xs)
+print(result)
+assert result is None
+print("Success!")
diff --git a/users/wpcarro/scratch/facebook/language.py b/users/wpcarro/scratch/facebook/language.py
new file mode 100644
index 0000000000..b57f469b49
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/language.py
@@ -0,0 +1,70 @@
+import random
+
+# Write an evaluator for a small language:
+#   - operators: '+', '*'
+#   - operands:  Integers
+#
+# E.g. evaluate("2+14*90+5*16")
+
+def tokenize(xs):
+    result = []
+    i = 0
+    while i < len(xs):
+        current = xs[i]
+        if current in {'*', '+'}:
+            result.append(current)
+            i += 1
+            continue
+        elif current == ' ':
+            i += 1
+            continue
+        else:
+            i += 1
+            while i < len(xs) and xs[i] in {str(x) for x in range(10)}:
+                current += xs[i]
+                i += 1
+            result.append(int(current))
+    return result
+
+def ast(tokens):
+    result = []
+    series = []
+    for token in tokens:
+        if token == '+':
+            result.append(series)
+            series = []
+        elif token == '*':
+            continue
+        else:
+            series.append(token)
+    if series:
+        result.append(series)
+    return result
+
+def product(xs):
+    result = 1
+    for x in xs:
+        result *= x
+    return result
+
+def evaluate(x):
+    tokens = tokenize(x)
+    tree = ast(tokens)
+    return sum([product(xs) for xs in tree])
+
+n = 7
+operands = [random.randint(0, 100) for _ in range(n)]
+operators = [random.choice(['+','*']) for _ in range(n - 1)]
+expr = []
+for i in range(n - 1):
+    expr.append(operands[i])
+    expr.append(operators[i])
+expr.append(operands[-1])
+
+expr = ' '.join([str(x) for x in expr])
+print("Expression: {}".format(expr))
+print("Tokens: {}".format(tokenize(expr)))
+print("AST: {}".format(ast(tokenize(expr))))
+print("Answer: {}".format(evaluate(expr)))
+assert evaluate(expr) == eval(expr)
+print("Success!")
diff --git a/users/wpcarro/scratch/facebook/language2.py b/users/wpcarro/scratch/facebook/language2.py
new file mode 100644
index 0000000000..3aebd45483
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/language2.py
@@ -0,0 +1,50 @@
+
+
+
+
+
+
+
+
+
+def tokenize(xs):
+    result = []
+    i = 0
+    while i < len(xs):
+        curr = xs[i]
+        if curr in {'*','+'}:
+            result.append(curr)
+            i += 1
+            continue
+        i += 1
+        while i < len(xs) and xs[i] in {str(x) for x in range(10)}:
+            curr += xs[i]
+            i += 1
+        result.append(int(curr))
+    return result
+
+def parse(tokens):
+    result = []
+    series = []
+    for token in tokens:
+        if token == '*':
+            continue
+        elif token == '+':
+            result.append(series)
+            series = []
+        else:
+            series.append(token)
+    if series:
+        result.append(series)
+    return result
+
+def product(xs):
+    result = 1
+    for x in xs:
+        result *= x
+    return result
+
+def evaluate(tree):
+    return sum([product(xs) for xs in tree])
+
+print(evaluate(parse(tokenize("2+30*8*9+10"))))
diff --git a/users/wpcarro/scratch/facebook/largest-contiguous-sum.py b/users/wpcarro/scratch/facebook/largest-contiguous-sum.py
new file mode 100644
index 0000000000..7761bf1c61
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/largest-contiguous-sum.py
@@ -0,0 +1,15 @@
+def find_sum(xs):
+    result = float('-inf')
+    streak = 0
+    for x in xs:
+        result = max(result, streak, x)
+        if streak + x <= 0:
+            streak = x
+        else:
+            streak += x
+    return result
+
+
+x = [2,-8,3,-2,4,-10]
+assert find_sum(x) == 5
+print("Success!")
diff --git a/users/wpcarro/scratch/facebook/largest-stack.py b/users/wpcarro/scratch/facebook/largest-stack.py
new file mode 100644
index 0000000000..052db44153
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/largest-stack.py
@@ -0,0 +1,49 @@
+from stack import Stack, from_list
+from heapq import heapify, heappush, heappop
+from random import shuffle
+
+class MaxStack(Stack):
+    def __init__(self):
+        self.max = Stack()
+        super().__init__()
+
+    def __repr__(self):
+        return super().__repr__()
+
+    def push(self, x):
+        super().push(x)
+        max = self.get_max()
+        if not max:
+            self.max.push(x)
+        else:
+            self.max.push(max if x < max else x)
+
+    def pop(self):
+        self.max.pop()
+        return super().pop()
+
+    def get_max(self):
+        return self.max.peek()
+
+xs = list(range(1, 11))
+shuffle(xs)
+stack = MaxStack()
+for x in xs:
+    stack.push(x)
+
+print(stack)
+result = stack.get_max()
+print(result)
+assert result == 10
+
+popped = stack.pop()
+print("Popped: {}".format(popped))
+print(stack)
+while popped != 10:
+    assert stack.get_max() == 10
+    popped = stack.pop()
+    print("Popped: {}".format(popped))
+    print(stack)
+
+assert stack.get_max() != 10
+print("Success!")
diff --git a/users/wpcarro/scratch/facebook/leetcode.org b/users/wpcarro/scratch/facebook/leetcode.org
new file mode 100644
index 0000000000..6e915faf29
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/leetcode.org
@@ -0,0 +1,163 @@
+# This list is from:
+# https://www.teamblind.com/post/New-Year-Gift---Curated-List-of-Top-100-LeetCode-Questions-to-Save-Your-Time-OaM1orEU
+* Array
+** DONE Two Sum
+   https://leetcode.com/problems/two-sum/
+** DONE Best Time to Buy and Sell Stock
+   https://leetcode.com/problems/best-time-to-buy-and-sell-stock/
+** DONE Contains Duplicate
+   https://leetcode.com/problems/contains-duplicate/
+** DONE Product of Array Except Self
+   https://leetcode.com/problems/product-of-array-except-self/
+** DONE Maximum Subarray
+   https://leetcode.com/problems/maximum-subarray/
+** DONE Maximum Product Subarray
+   https://leetcode.com/problems/maximum-product-subarray/
+** DONE Find Minimum in Rotated Sorted Array
+   https://leetcode.com/problems/find-minimum-in-rotated-sorted-array/
+** DONE Search in Rotated Sorted Array
+   https://leetcode.com/problems/search-in-rotated-sorted-array/
+** DONE 3Sum
+   https://leetcode.com/problems/3sum/
+** DONE Container With Most Water
+   https://leetcode.com/problems/container-with-most-water/
+* Binary
+** DONE Sum of Two Integers
+   https://leetcode.com/problems/sum-of-two-integers/
+** DONE Number of 1 Bits
+   https://leetcode.com/problems/number-of-1-bits/
+** TODO Counting Bits
+   https://leetcode.com/problems/counting-bits/
+** DONE Missing Number
+   https://leetcode.com/problems/missing-number/
+** TODO Reverse Bits
+   https://leetcode.com/problems/reverse-bits/
+* Dynamic Programming
+** DONE Climbing Stairs
+   https://leetcode.com/problems/climbing-stairs/
+** TODO Coin Change
+   https://leetcode.com/problems/coin-change/
+** TODO Longest Increasing Subsequence
+   https://leetcode.com/problems/longest-increasing-subsequence/
+** TODO Longest Common Subsequence
+** DONE Word Break Problem
+   https://leetcode.com/problems/word-break/
+** TODO Combination Sum
+   https://leetcode.com/problems/combination-sum-iv/
+** TODO House Robber
+   https://leetcode.com/problems/house-robber/
+** TODO House Robber II
+   https://leetcode.com/problems/house-robber-ii/
+** TODO Decode Ways
+   https://leetcode.com/problems/decode-ways/
+** TODO Unique Paths
+   https://leetcode.com/problems/unique-paths/
+** TODO Jump Game
+   https://leetcode.com/problems/jump-game/
+* Graph
+** DONE Clone Graph
+   https://leetcode.com/problems/clone-graph/
+** DONE Course Schedule
+   https://leetcode.com/problems/course-schedule/
+** TODO Pacific Atlantic Water Flow
+   https://leetcode.com/problems/pacific-atlantic-water-flow/
+** DONE Number of Islands
+   https://leetcode.com/problems/number-of-islands/
+** TODO Longest Consecutive Sequence
+   https://leetcode.com/problems/longest-consecutive-sequence/
+** TODO Alien Dictionary (Leetcode Premium)
+   https://leetcode.com/problems/alien-dictionary/
+** DONE Graph Valid Tree (Leetcode Premium)
+   https://leetcode.com/problems/graph-valid-tree/
+** DONE Number of Connected Components in an Undirected Graph (Leetcode Premium)
+   https://leetcode.com/problems/number-of-connected-components-in-an-undirected-graph/
+* Interval
+** TODO Insert Interval
+   https://leetcode.com/problems/insert-interval/
+** DONE Merge Intervals
+   https://leetcode.com/problems/merge-intervals/
+** TODO No Overlapping Intervals
+   https://leetcode.com/problems/non-overlapping-intervals/
+** DONE Meeting Rooms (Leetcode Premium)
+   https://leetcode.com/problems/meeting-rooms/
+** TODO Meeting Rooms II (Leetcode Premium)
+   https://leetcode.com/problems/meeting-rooms-ii/
+* Linked List
+** DONE Reverse a Linked List
+   https://leetcode.com/problems/reverse-linked-list/
+** DONE Detect Cycle in a Linked List
+   https://leetcode.com/problems/linked-list-cycle/
+** DONE Merge Two Sorted Lists
+   https://leetcode.com/problems/merge-two-sorted-lists/
+** DONE Merge K Sorted Lists
+   https://leetcode.com/problems/merge-k-sorted-lists/
+** DONE Remove Nth Node From End Of List
+   https://leetcode.com/problems/remove-nth-node-from-end-of-list/
+** DONE Reorder List
+   https://leetcode.com/problems/reorder-list/
+* Matrix
+** DONE Set Matrix Zeroes
+   https://leetcode.com/problems/set-matrix-zeroes/
+** DONE Spiral Matrix
+   https://leetcode.com/problems/spiral-matrix/
+** TODO Rotate Image
+   https://leetcode.com/problems/rotate-image/
+** DONE Word Search
+   https://leetcode.com/problems/word-search/
+* String
+** TODO Longest Substring Without Repeating Characters
+   https://leetcode.com/problems/longest-substring-without-repeating-characters/
+** TODO Longest Repeating Character Replacement
+   https://leetcode.com/problems/longest-repeating-character-replacement/
+** TODO Minimum Window Substring
+   https://leetcode.com/problems/minimum-window-substring/
+** DONE Valid Anagram
+   https://leetcode.com/problems/valid-anagram/
+** DONE Group Anagrams
+   https://leetcode.com/problems/group-anagrams/
+** DONE Valid Parentheses
+   https://leetcode.com/problems/valid-parentheses/
+** DONE Valid Palindrome
+   https://leetcode.com/problems/valid-palindrome/
+** TODO Longest Palindromic Substring
+   https://leetcode.com/problems/longest-palindromic-substring/
+** TODO Palindromic Substrings
+   https://leetcode.com/problems/palindromic-substrings/
+** DONE Encode and Decode Strings (Leetcode Premium)
+   https://leetcode.com/problems/encode-and-decode-strings/
+* Tree
+** DONE Maximum Depth of Binary Tree
+   https://leetcode.com/problems/maximum-depth-of-binary-tree/
+** DONE Same Tree
+   https://leetcode.com/problems/same-tree/
+** DONE Invert/Flip Binary Tree
+   https://leetcode.com/problems/invert-binary-tree/
+** DONE Binary Tree Maximum Path Sum
+   https://leetcode.com/problems/binary-tree-maximum-path-sum/
+** DONE Binary Tree Level Order Traversal
+   https://leetcode.com/problems/binary-tree-level-order-traversal/
+** DONE Serialize and Deserialize Binary Tree
+   https://leetcode.com/problems/serialize-and-deserialize-binary-tree/
+** DONE Subtree of Another Tree
+   https://leetcode.com/problems/subtree-of-another-tree/
+** DONE Construct Binary Tree from Preorder and Inorder Traversal
+   https://leetcode.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal/
+** DONE Validate Binary Search Tree
+   https://leetcode.com/problems/validate-binary-search-tree/
+** DONE Kth Smallest Element in a BST
+   https://leetcode.com/problems/kth-smallest-element-in-a-bst/
+** DONE Lowest Common Ancestor of BST
+   https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-search-tree/
+** DONE Implement Trie (Prefix Tree)
+   https://leetcode.com/problems/implement-trie-prefix-tree/
+** DONE Add and Search Word
+   https://leetcode.com/problems/add-and-search-word-data-structure-design/
+** DONE Word Search II
+   https://leetcode.com/problems/word-search-ii/
+* Heap
+** DONE Merge K Sorted Lists
+   https://leetcode.com/problems/merge-k-sorted-lists/
+** DONE Top K Frequent Elements
+   https://leetcode.com/problems/top-k-frequent-elements/
+** DONE Find Median from Data Stream
+   https://leetcode.com/problems/find-median-from-data-stream/
diff --git a/users/wpcarro/scratch/facebook/linked-list-cycles.py b/users/wpcarro/scratch/facebook/linked-list-cycles.py
new file mode 100644
index 0000000000..56f54d4978
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/linked-list-cycles.py
@@ -0,0 +1,26 @@
+import random
+
+from linked_list import Node
+
+def contains_cycle(node):
+    one = node
+    two = node
+    while two.next and two.next.next:
+        one = one.next
+        two = two.next.next
+        if one == two:
+            return True
+    return False
+
+xs = Node(1, Node(2, Node(3)))
+assert not contains_cycle(xs)
+print("Success!")
+
+a = Node(1)
+b = Node(2)
+c = Node(3)
+a.next = b
+b.next = c
+c.next = random.choice([a, b, c])
+assert contains_cycle(a)
+print("Success!")
diff --git a/users/wpcarro/scratch/facebook/linked_list.py b/users/wpcarro/scratch/facebook/linked_list.py
new file mode 100644
index 0000000000..1ae7061e83
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/linked_list.py
@@ -0,0 +1,22 @@
+class Node(object):
+    def __init__(self, value=None, next=None):
+        self.value = value
+        self.next = next
+
+    def __repr__(self):
+        result = []
+        node = self
+        while node:
+            result.append(str(node.value))
+            node = node.next
+        return 'LinkedList({xs})'.format(xs=', '.join(result))
+
+def from_list(xs):
+    head = Node(xs[0])
+    node = head
+    for x in xs[1:]:
+        node.next = Node(x)
+        node = node.next
+    return head
+
+list = from_list(['A', 'B', 'C'])
diff --git a/users/wpcarro/scratch/facebook/london-knapsack.py b/users/wpcarro/scratch/facebook/london-knapsack.py
new file mode 100644
index 0000000000..a034fb4961
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/london-knapsack.py
@@ -0,0 +1,42 @@
+from utils import get, init_table, print_table
+
+def optimal_itinerary(duration, items):
+    min_duration = min([duration for duration, _ in items])
+    max_duration = max([duration for duration, _ in items])
+    table = init_table(rows=len(items), cols=int(max_duration / min_duration), default=0)
+    to_index = lambda duration: int(duration / min_duration) - 1
+    to_duration = lambda i: i * min_duration + min_duration
+
+    for row in range(len(table)):
+        for col in range(len(table[row])):
+            curr_duration = to_duration(col)
+            duration, value = items[row]
+            if duration > curr_duration:
+                a = 0
+            else:
+                a = value + get(table, row - 1, to_index(curr_duration - duration))
+            b = get(table, row - 1, col)
+            table[row][col] = max([a, b])
+
+        print_table(table)
+    return table[-1][-1]
+
+# You're in London for two days, and you'd like to see the following
+# attractions. How can you maximize your time spent in London?
+westminster = (0.5, 7)
+globe_theater = (0.5, 6)
+national_gallery = (1, 9)
+british_museum = (2, 9)
+st_pauls_cathedral = (0.5, 8)
+items = [
+    westminster,
+    globe_theater,
+    national_gallery,
+    british_museum,
+    st_pauls_cathedral,
+]
+result = optimal_itinerary(2, items)
+expected = 24
+print(result, expected)
+assert result == expected
+print("Success!")
diff --git a/users/wpcarro/scratch/facebook/longest-common-substring.py b/users/wpcarro/scratch/facebook/longest-common-substring.py
new file mode 100644
index 0000000000..8a838db45d
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/longest-common-substring.py
@@ -0,0 +1,20 @@
+from utils import get, init_table, print_table
+
+def longest_common_substring(a, b):
+    """
+    Computes the length of the longest string that's present in both `a` and
+    `b`.
+    """
+    table = init_table(rows=len(b), cols=len(a), default=0)
+    for row in range(len(table)):
+        for col in range(len(table[row])):
+            if b[row] == a[col]:
+                table[row][col] = 1 + get(table, row - 1, col - 1)
+    return max([max(row) for row in table])
+
+dictionary = ["fish", "vista"]
+result = [longest_common_substring("hish", x) for x in dictionary]
+expected = [3, 2]
+print(result, expected)
+assert result == expected
+print("Success!")
diff --git a/users/wpcarro/scratch/facebook/merge-sorted-arrays.py b/users/wpcarro/scratch/facebook/merge-sorted-arrays.py
new file mode 100644
index 0000000000..ae9377ad11
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/merge-sorted-arrays.py
@@ -0,0 +1,44 @@
+def merge_sorted(xs, ys):
+    result = []
+    i, j = 0, 0
+
+    while i < len(xs) and j < len(ys):
+        if xs[i] <= ys[j]:
+            result.append(xs[i])
+            i += 1
+        else:
+            result.append(ys[j])
+            j += 1
+
+    while i < len(xs):
+        result.append(xs[i])
+        i += 1
+
+    while j < len(ys):
+        result.append(ys[j])
+        j += 1
+
+    return result
+
+# A
+result = merge_sorted([3, 4, 6, 10, 11, 15], [1, 5, 8, 12, 14, 19])
+print(result)
+assert result == [1, 3, 4, 5, 6, 8, 10, 11, 12, 14, 15, 19]
+
+# B
+result = merge_sorted([], [1,2,3])
+print(result)
+assert result == [1,2,3]
+
+# C
+result = merge_sorted([1,2,3], [])
+print(result)
+assert result == [1,2,3]
+
+# D
+result = merge_sorted([], [])
+print(result)
+assert result == []
+
+# Wahoo!
+print("Success!")
diff --git a/users/wpcarro/scratch/facebook/merging-ranges.py b/users/wpcarro/scratch/facebook/merging-ranges.py
new file mode 100644
index 0000000000..6da44572ee
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/merging-ranges.py
@@ -0,0 +1,23 @@
+
+def merge(xs):
+    xs.sort()
+    result = xs[0:1]
+    for a, b in xs[1:]:
+        y, z = result[-1]
+        if a <= z:
+            result[-1] = (y, max(b, z))
+        else:
+            result.append((a, b))
+    return result
+
+inputs = [([(0,1),(3,5),(4,8),(10,12),(9,10)], [(0,1),(3,8),(9,12)]),
+          ([(1,2),(2,3)], [(1,3)]),
+          ([(1,5),(2,3)], [(1,5)]),
+          ([(1,10),(2,6),(3,5),(7,9)], [(1,10)]),
+          ]
+for x, expected in inputs:
+    result = merge(x)
+    print(x)
+    print(result)
+    assert result == expected
+    print("Success!")
diff --git a/users/wpcarro/scratch/facebook/mesh-message.py b/users/wpcarro/scratch/facebook/mesh-message.py
new file mode 100644
index 0000000000..8438b059d8
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/mesh-message.py
@@ -0,0 +1,40 @@
+from heapq import heappush, heappop
+import random
+
+def shortest_path(a, b, graph):
+    seen = set()
+    h = []
+    heappush(h, (0, a, [a]))
+    while h:
+        km, x, path = heappop(h)
+        if x == b:
+            return path
+        for c in graph[x]:
+            if c not in seen:
+                heappush(h, (km + 1, c, path + [c]))
+    raise Exception("We were unable to find a path from {} to {}".format(a, b))
+
+graph = {
+    'Min'     : ['William', 'Jayden', 'Omar'],
+    'William' : ['Min', 'Noam'],
+    'Jayden'  : ['Min', 'Amelia', 'Ren', 'Noam'],
+    'Ren'     : ['Jayden', 'Omar'],
+    'Amelia'  : ['Jayden', 'Adam', 'Miguel'],
+    'Adam'    : ['Amelia', 'Miguel', 'Sofia', 'Lucas'],
+    'Miguel'  : ['Amelia', 'Adam', 'Liam', 'Nathan'],
+    'Noam'    : ['Nathan', 'Jayden', 'William'],
+    'Omar'    : ['Ren', 'Min', 'Scott'],
+    'Liam'    : ['Ren'],
+    'Nathan'  : ['Noam'],
+    'Scott'   : [],
+}
+
+result = shortest_path('Jayden', 'Adam', graph)
+print(result)
+assert result == ['Jayden', 'Amelia', 'Adam']
+print('Success!')
+
+beg = random.choice(list(graph.keys()))
+end = random.choice(list(graph.keys()))
+print("Attempting to find the shortest path between {} and {}".format(beg, end))
+print(shortest_path(beg, end, graph))
diff --git a/users/wpcarro/scratch/facebook/moderate/decompress-xml.py b/users/wpcarro/scratch/facebook/moderate/decompress-xml.py
new file mode 100644
index 0000000000..b22983ed7a
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/moderate/decompress-xml.py
@@ -0,0 +1,98 @@
+import string
+from parser import Parser
+
+mapping = {
+    1: "family",
+    2: "person",
+    3: "firstName",
+    4: "lastName",
+    5: "state",
+}
+
+def parse_int(i, xs):
+    result = ""
+    while i < len(xs) and xs[i] in string.digits:
+        result += xs[i]
+        i += 1
+    return i, int(result)
+
+def parse_string(i, xs):
+    result = ""
+    while xs[i+1] not in string.digits:
+        result += xs[i]
+        i += 1
+    return i, result
+
+def tokenize(xs):
+    result = []
+    i = 0
+    while i < len(xs):
+        if xs[i] in string.digits:
+            i, n = parse_int(i, xs)
+            result.append(n)
+        elif xs[i] in string.ascii_letters:
+            i, x = parse_string(i, xs)
+            result.append(x)
+        elif xs[i] == " ":
+            i += 1
+            continue
+    return result
+
+def parse(xs):
+    parser = Parser(tokenize(xs))
+    return parse_element(parser)
+
+# Element   -> Tag Attribute* End Element* End ;
+# Tag       -> INTEGER ;
+# Value     -> STRING End ;
+# Attribute -> Tag Value ;
+# End       -> 0 ;
+
+def parse_element(parser):
+    if type(parser.curr()) == str:
+        return parser.consume()
+    tag_id = parser.expect_predicate(lambda x: type(x) == int)
+    tag = mapping[tag_id]
+    attrs = parse_attrs(parser)
+    parser.expect([0])
+    children = []
+    while not parser.exhausted() and parser.curr() != 0:
+        children.append(parse_element(parser))
+    parser.expect([0])
+    return [tag, attrs, children]
+
+def parse_attrs(parser):
+    result = []
+    while parser.curr() != 0:
+        tag_id = parser.expect_predicate(lambda x: type(x) == int)
+        tag = mapping[tag_id]
+        value = parser.consume()
+        result.append((tag, value))
+    return result
+
+def stringify_xml(tree, indent=0):
+    if type(tree) == str:
+        return tree
+    result = ""
+    tag, attrs, children = tree
+
+    str_attrs = []
+    for k, v in attrs:
+        str_attrs.append("{}=\"{}\"".format(k, v))
+    str_attrs = (" " if str_attrs else "") + " ".join(str_attrs)
+
+    str_children = []
+    for child in children:
+        str_children.append(" " * 2 * indent + stringify_xml(child, indent + 1))
+    str_children = "\n".join(str_children)
+
+    result += "{}<{}{}>\n{}{}\n{}</{}>".format(
+        " " * 2 * indent, tag, str_attrs, " " * 2 * indent, str_children,
+        " " * 2 * indent, tag)
+    return result
+
+x = "1 4 McDowell 5 CA 0 2 3 Gayle 0 Some Message 0 0"
+print("Input:   {}".format(x))
+print("Tokens:  {}".format(tokenize(x)))
+print("Parsed:  {}".format(parse(x)))
+print("{}".format(stringify_xml(parse(x))))
diff --git a/users/wpcarro/scratch/facebook/moderate/find-pairs-for-sum.py b/users/wpcarro/scratch/facebook/moderate/find-pairs-for-sum.py
new file mode 100644
index 0000000000..69c2fc4312
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/moderate/find-pairs-for-sum.py
@@ -0,0 +1,19 @@
+import random
+
+def find_pairs(xs, n):
+    """
+    Return all pairs of integers in `xs` that sum to `n`.
+    """
+    seeking = set()
+    result = set()
+    for x in xs:
+        if x in seeking:
+            result.add((n - x, x))
+        else:
+            seeking.add(n - x)
+    return result
+
+xs = [random.randint(1, 10) for _ in range(10)]
+n = random.randint(1, 10) + random.randint(1, 10)
+print("Seeking all pairs in {} for {}...".format(xs, n))
+print(find_pairs(xs, n))
diff --git a/users/wpcarro/scratch/facebook/moderate/parser.py b/users/wpcarro/scratch/facebook/moderate/parser.py
new file mode 100644
index 0000000000..57dfb058c0
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/moderate/parser.py
@@ -0,0 +1,37 @@
+class Parser(object):
+    def __init__(self, tokens):
+        self.tokens = tokens
+        self.i = 0
+
+    def prev(self):
+        return self.tokens[self.i - 1]
+
+    def curr(self):
+        return self.tokens[self.i]
+
+    def next(self):
+        return self.tokens[self.i + 1]
+
+    def consume(self):
+        if not self.exhausted():
+            self.i += 1
+            return self.prev()
+
+    def match(self, xs):
+        if not self.exhausted() and self.curr() in xs:
+            self.consume()
+            return True
+        return False
+
+    def expect(self, xs):
+        if not self.match(xs):
+            raise Exception("Expected token \"{}\" but received \"{}\"".format(xs, self.curr()))
+        return self.prev()
+
+    def expect_predicate(self, predicate):
+        if predicate(self.curr()):
+            return self.consume()
+        raise Exception("Expected token \"{}\" to pass predicate, but it did not".format(self.curr()))
+
+    def exhausted(self):
+        return self.i >= len(self.tokens)
diff --git a/users/wpcarro/scratch/facebook/moderate/rand7.py b/users/wpcarro/scratch/facebook/moderate/rand7.py
new file mode 100644
index 0000000000..ed3a7cea80
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/moderate/rand7.py
@@ -0,0 +1,25 @@
+# Define a function, rand7, that generates a random number [0,7), using only
+# rand5, which generates a random number [0,5).
+
+import random
+from collections import Counter
+
+# Returns [0,4]
+def rand5():
+    return random.randint(0,4)
+
+# Return [0,6]
+def rand7_a():
+    return sum(rand5() for _ in range(7)) % 7
+
+# Return [0,6]
+def rand7_b():
+    x = 5 * rand5() + rand5()
+    if x < 21:
+        return x % 7
+    return rand7_b()
+
+c = Counter([rand7_a() for _ in range(100000)])
+print(c)
+c = Counter([rand7_b() for _ in range(100000)])
+print(c)
diff --git a/users/wpcarro/scratch/facebook/moderate/tic-tac-toe-checker.py b/users/wpcarro/scratch/facebook/moderate/tic-tac-toe-checker.py
new file mode 100644
index 0000000000..342c29be6b
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/moderate/tic-tac-toe-checker.py
@@ -0,0 +1,99 @@
+import random
+
+def print_board(board):
+    result = []
+    for row in range(len(board)):
+        r = []
+        for col in range(len(board[row])):
+            cell = board[row][col]
+            if not cell:
+                r.append("-")
+            else:
+                r.append(cell)
+        result.append(" | ".join(r))
+    print("\n---------\n".join(result))
+
+def init_board():
+    result = []
+    for row in range(3):
+        r = []
+        for col in range(3):
+            r.append(None)
+        result.append(r)
+    return result
+
+def check(board, player):
+    print_board(board)
+    print()
+    if player not in "XO":
+        raise Exception("Only checking the board for Xs or Os. You supplied {}".format(player))
+    dn, ax, ddg, udg = "DOWN", "ACROSS", "DOWN_DIAGONAL", "UP_DIAGONAL"
+    ways = [
+        [[dn, ax, ddg], [dn], [dn, udg]],
+        [[ax], [], []],
+        [[ax], [], []],
+    ]
+    for row in range(len(board)):
+        for col in range(len(board[row])):
+            if board[row][col] == player:
+                xs = ways[row][col]
+                for x in xs:
+                    if x == dn:
+                        if {player} == {board[row+1][col], board[row+2][col]}:
+                            return True
+                    if x == ax:
+                        if {player} == {board[row][col+1], board[row][col+2]}:
+                            return True
+                    if x == ddg:
+                        if {player} == {board[row+1][col+1], board[row+2][col+2]}:
+                            return True
+                    if x == udg:
+                        if {player} == {board[row+1][col-1], board[row+2][col-2]}:
+                            return True
+    return False
+
+def op(player):
+    return "X" if player == "O" else "O"
+
+dn_win = lambda p: [
+    [op(p), p, None],
+    [op(p), p, None],
+    [None,  p, None],
+]
+
+ax_win = lambda p: [
+    [p, p, p],
+    [op(p), op(p), None],
+    [None, None, None],
+]
+
+ddg_win = lambda p: [
+    [p, None, None],
+    [op(p), p, None],
+    [op(p), None, p],
+]
+
+udg_win = lambda p: [
+    [op(p), None, p],
+    [op(p), p, None],
+    [p, None, None],
+]
+
+# Down
+p = random.choice(["X", "O"])
+assert check(dn_win(p), p) == True
+assert check(dn_win(p), op(p)) == False
+# Across
+p = random.choice(["X", "O"])
+assert check(ax_win(p), p) == True
+assert check(ax_win(p), op(p)) == False
+# Down Diagonally
+p = random.choice(["X", "O"])
+assert check(ddg_win(p), p) == True
+assert check(ddg_win(p), op(p)) == False
+# Down Diagonally
+p = random.choice(["X", "O"])
+assert check(udg_win(p), p) == True
+assert check(udg_win(p), op(p)) == False
+# Success
+print("Tests pass!")
diff --git a/users/wpcarro/scratch/facebook/moderate/unsorted-substring.py b/users/wpcarro/scratch/facebook/moderate/unsorted-substring.py
new file mode 100644
index 0000000000..de7326b058
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/moderate/unsorted-substring.py
@@ -0,0 +1,67 @@
+# Write a function that accepts an array of integers and returns the indices for
+# the starting and ending integers that, if their elements were sorted, the
+# entire array would be sorted.
+
+################################################################################
+# First Attempt
+################################################################################
+
+def unsorted_substring(xs):
+    ys = xs[:]; ys.sort()
+    m = 0
+    while xs[m] == ys[m]:
+        m += 1
+        if m >= len(xs):
+            return -1, -1
+    n = len(xs) - 1
+    while xs[n] == ys[n]:
+        n -= 1
+    return m, n
+
+################################################################################
+# Second Attempt
+################################################################################
+
+def unsorted_substring_2(xs):
+    beg = 1
+    while xs[beg - 1] <= xs[beg]:
+        beg += 1
+        if beg >= len(xs):
+            return -1, -1
+    end = len(xs) - 2
+    while xs[end + 1] >= xs[end]:
+        end -= 1
+
+    min_mid = xs[beg]
+    max_mid = xs[beg]
+    i = beg + 1
+    while i <= end:
+        min_mid = min(min_mid, xs[i])
+        max_mid = max(max_mid, xs[i])
+        i += 1
+
+    # beg -= 1 until max(lhs) <= min(mid)
+    while beg - 1 >= 0 and xs[beg - 1] >= min_mid:
+        beg -= 1
+
+    # end += 1 while max(mid) <= min(rhs)
+    while end + 1 < len(xs) and max_mid >= xs[end + 1]:
+        end += 1
+    return beg, end
+
+################################################################################
+# Tests
+################################################################################
+
+xs = [
+    [1,2,4,7,10,11,7,12,6,7,16,18,19],
+    [1,2,3,4],
+    [4,3,2,1],
+    [1,3,2,4],
+    [2,1,3,4],
+]
+
+for x in xs:
+    print("Testing: {}".format(x))
+    print("1) {}".format(unsorted_substring(x)))
+    print("2) {}".format(unsorted_substring_2(x)))
diff --git a/users/wpcarro/scratch/facebook/move-zeroes-to-end.py b/users/wpcarro/scratch/facebook/move-zeroes-to-end.py
new file mode 100644
index 0000000000..1535b5a9fa
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/move-zeroes-to-end.py
@@ -0,0 +1,62 @@
+from collections import deque
+
+def move_zeroes_to_end_quadratic(xs):
+    """
+    This solution is suboptimal. It runs in quadratic time, and it uses constant
+    space.
+    """
+    i = 0
+    while i < len(xs) - 1:
+        if xs[i] == 0:
+            j = i + 1
+            while j < len(xs) and xs[j] == 0:
+                j += 1
+            if j >= len(xs):
+                break
+            xs[i], xs[j] = xs[j], xs[i]
+        i += 1
+
+def move_zeroes_to_end_linear(xs):
+    """
+    This solution is clever. It runs in linear time proportionate to the number
+    of elements in `xs`, and has linear space proportionate to the number of
+    consecutive zeroes in `xs`.
+    """
+    q = deque()
+    for i in range(len(xs)):
+        if xs[i] == 0:
+            q.append(i)
+        else:
+            if q:
+                j = q.popleft()
+                xs[i], xs[j] = xs[j], xs[i]
+                q.append(i)
+
+def move_zeroes_to_end_linear_constant_space(xs):
+    """
+    This is the optimal solution. It runs in linear time and uses constant
+    space.
+    """
+    i = 0
+    for j in range(len(xs)):
+        if xs[j] != 0:
+            xs[i], xs[j] = xs[j], xs[i]
+            i += 1
+
+
+################################################################################
+# Tests
+################################################################################
+
+xss = [
+    [1, 2, 0, 3, 4, 0, 0, 5, 0],
+    [0, 1, 2, 0, 3, 4],
+    [0, 0],
+]
+
+f = move_zeroes_to_end_linear_constant_space
+
+for xs in xss:
+    print(xs)
+    f(xs)
+    print(xs)
diff --git a/users/wpcarro/scratch/facebook/mst.py b/users/wpcarro/scratch/facebook/mst.py
new file mode 100644
index 0000000000..81aa5cd487
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/mst.py
@@ -0,0 +1,71 @@
+from heapq import heappush, heappop
+import random
+
+def to_vertex_list(graph):
+    result = {}
+    for a, b, kg in graph:
+        if a in result:
+            result[a].append((b, kg))
+        else:
+            result[a] = [(b, kg)]
+        if b in result:
+            result[b].append((a, kg))
+        else:
+            result[b] = [(a, kg)]
+    return result
+
+def mst(graph):
+    graph = to_vertex_list(graph)
+    beg = random.choice(list(graph.keys()))
+    h = []
+    result = []
+    seen = set()
+    for c, kg in graph[beg]:
+        heappush(h, (kg, beg, c))
+    while h:
+        kg, beg, end = heappop(h)
+        # detect cycles
+        if end in seen:
+            continue
+        # use the edge
+        seen.add(beg)
+        seen.add(end)
+        result.append((beg, end))
+        for c, kg in graph[end]:
+            heappush(h, (kg, end, c))
+    return result
+
+graphs = [
+    [
+        ('A', 'B', 7),
+        ('A', 'D', 5),
+        ('B', 'D', 9),
+        ('E', 'D', 15),
+        ('F', 'D', 6),
+        ('F', 'G', 11),
+        ('F', 'E', 8),
+        ('G', 'E', 9),
+        ('C', 'E', 5),
+        ('B', 'E', 7),
+        ('B', 'C', 8),
+    ],
+    [
+        ('A', 'B', 4),
+        ('A', 'C', 8),
+        ('B', 'C', 11),
+        ('B', 'E', 8),
+        ('C', 'D', 7),
+        ('C', 'F', 1),
+        ('D', 'E', 2),
+        ('D', 'F', 6),
+        ('E', 'G', 7),
+        ('E', 'H', 4),
+        ('F', 'H', 2),
+        ('G', 'H', 14),
+        ('G', 'I', 9),
+        ('H', 'I', 10),
+    ],
+]
+
+for graph in graphs:
+    print(mst(graph))
diff --git a/users/wpcarro/scratch/facebook/n-queens.py b/users/wpcarro/scratch/facebook/n-queens.py
new file mode 100644
index 0000000000..fc9326886c
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/n-queens.py
@@ -0,0 +1,46 @@
+def print_board(board):
+    result = []
+    for row in range(8):
+        r = []
+        for col in range(8):
+            r.append("X" if col == board[row] else "-")
+        result.append(" ".join(r))
+    print("\n".join(result))
+    print()
+
+def can_place(board, row, col):
+    column_occupied = not any([board[i] == col for i in range(row)])
+
+    diagonals_clear = True
+    for r in range(row):
+        w = abs(col - board[r])
+        h = abs(r - row)
+        if w == h:
+            diagonals_clear = False
+            break
+
+    return all([column_occupied, diagonals_clear])
+
+def init_board():
+    board = []
+    for row in range(8):
+        board.append(None)
+    return board
+
+def copy_board(board):
+    return board[:]
+
+def n_queens():
+    do_n_queens(init_board(), 0, 0)
+
+def do_n_queens(board, row, col):
+    if row == 8:
+        print_board(board)
+        return
+    for i in range(col, 8):
+        if can_place(board, row, i):
+            copy = copy_board(board)
+            copy[row] = i
+            do_n_queens(copy, row + 1, 0)
+
+n_queens()
diff --git a/users/wpcarro/scratch/facebook/nearby-words.py b/users/wpcarro/scratch/facebook/nearby-words.py
new file mode 100644
index 0000000000..d2fc3cf5cf
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/nearby-words.py
@@ -0,0 +1,33 @@
+def nearby_chars(c):
+    keyboard = [
+        "qwertyuiop",
+        "asdfghjkl",
+        "zxcvbnm",
+    ]
+
+    for row in keyboard:
+        for i in range(len(row)):
+            if row[i] == c:
+                result = set()
+                if i + 1 < len(row):
+                    result.add(row[i + 1])
+                if i - 1 >= 0:
+                    result.add(row[i - 1])
+                return result
+
+def is_word(word):
+    words = {
+        "hello",
+    }
+    return word in words
+
+def nearby_words(x):
+    result = set()
+    for i in range(len(x)):
+        for c in nearby_chars(x[i]):
+            candidate = x[0:i] + c + x[i+1:]
+            if is_word(candidate):
+                result.add(candidate)
+    return result
+
+print(nearby_words('gello'))
diff --git a/users/wpcarro/scratch/facebook/node.py b/users/wpcarro/scratch/facebook/node.py
new file mode 100644
index 0000000000..4e24983af7
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/node.py
@@ -0,0 +1,38 @@
+class Node(object):
+    def __init__(self, value, left=None, right=None):
+        self.value = value
+        self.left = left
+        self.right = right
+
+    def insert_left(self, value):
+        self.left = Node(value)
+        return self.left
+
+    def insert_right(self, value):
+        self.right = Node(value)
+        return self.right
+
+tree = Node(
+    50,
+    Node(
+        17,
+        Node(
+            12,
+            Node(9),
+            Node(14),
+        ),
+        Node(
+            23,
+            Node(19),
+        ),
+    ),
+    Node(
+        72,
+        Node(
+            54,
+            None,
+            Node(67)
+        ),
+        Node(76),
+    ),
+)
diff --git a/users/wpcarro/scratch/facebook/nth-fibonacci.py b/users/wpcarro/scratch/facebook/nth-fibonacci.py
new file mode 100644
index 0000000000..f524067b3b
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/nth-fibonacci.py
@@ -0,0 +1,13 @@
+# 0, 1, 1, 2, 3, 5
+def fib(n):
+    if n < 0:
+        raise Exception("Need to supply an index that's >= 0. Not: {}".format(n))
+    elif n in {0, 1}:
+        return n
+    state = [0, 1]
+    for i in range(1, n):
+        state[0], state[1] = state[1], state[0] + state[1]
+    return state[-1]
+
+for i in range(10):
+    print("fib({}) => {}".format(i, fib(i)))
diff --git a/users/wpcarro/scratch/facebook/onsite.txt b/users/wpcarro/scratch/facebook/onsite.txt
new file mode 100644
index 0000000000..b5242c4bd3
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/onsite.txt
@@ -0,0 +1,22 @@
+** Behavior Interview **
+- Can I work in an unstructured environment?
+- Do I have a growth mindset?
+- How do I handle conflict?
+- Am I empathic?
+- Am I a self-starter?
+- What is my communication style?
+- Do I persevere?
+- <forgot to write this one down>
+
+** Design Interview **
+- requirement gathering, problem exploring
+- component analysis
+- quantitative analysis
+- trade-offs
+- bottlenecks, weaknesses
+- securing data (e.g. PII)
+
+Consider:
+- pagination
+- push/pull requests
+- API design
diff --git a/users/wpcarro/scratch/facebook/parsing/json.py b/users/wpcarro/scratch/facebook/parsing/json.py
new file mode 100644
index 0000000000..3975e973fe
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/parsing/json.py
@@ -0,0 +1,121 @@
+from parser import Parser
+
+# As an exercise to stress-test my understanding of recursive descent parsers,
+# I'm attempting to write a JSON parser without referencing any existing BNF
+# descriptions of JSON or existing JSON parser implementations.
+#
+# I'm only parsing a subset of JSON: enough to parse `sample`. Here is the BNF
+# that I wrote to describe my expected input:
+#
+# expression -> object
+# object     -> '{' ( STRING ':' expression ) ( ',' STRING ':' expression )* '}'
+#            |  array
+# array      -> '[' expression ( ',' expression )* ']'
+#            |  literal
+# literal    -> STRING | INT
+
+def tokenize(xs):
+    """
+    Return a list of tokens from the string input, `xs`.
+    """
+    result = []
+    i = 0
+    while i < len(xs):
+        # single characters
+        if xs[i] in ",{}:[]":
+            result.append(xs[i])
+            i += 1
+        # strings
+        elif xs[i] == "\"":
+            curr = xs[i]
+            i += 1
+            while xs[i] != "\"":
+                curr += xs[i]
+                i += 1
+            curr += xs[i]
+            result.append(curr)
+            i += 1
+        # integers
+        elif xs[i] in "0123456789":
+            curr = xs[i]
+            i += 1
+            while xs[i] in "0123456789":
+                curr += xs[i]
+                i += 1
+            result.append(int(curr))
+        # whitespace
+        elif xs[i] in {" ", "\n"}:
+            i += 1
+    return result
+
+def parse_json(x):
+    """
+    Attempt to parse the string, `x`, into JSON.
+    """
+    tokens = tokenize(x)
+    return parse_object(Parser(tokens))
+
+def parse_object(parser):
+    if parser.match(['{']):
+        key = parse_string(parser)
+        parser.expect([':'])
+        value = parse_object(parser)
+        result = [(key, value)]
+        while parser.match([',']):
+            key = parse_string(parser)
+            parser.match([':'])
+            value = parse_object(parser)
+            result.append((key, value))
+        return result
+    return parse_array(parser)
+
+def parse_array(parser):
+    if parser.match(['[']):
+        if parser.match([']']):
+            return []
+        result = [parse_object(parser)]
+        while parser.match([',']):
+            result.append(parse_object(parser))
+        parser.expect([']'])
+        return result
+    else:
+        return parse_literal(parser)
+
+def parse_string(parser):
+    if parser.curr().startswith("\""):
+        return parser.consume()
+    else:
+        raise Exception("Unexpected token: {}".format(parser.curr()))
+
+def parse_literal(parser):
+    return parser.consume()
+
+sample = """
+{
+  "glossary": {
+    "title": "example glossary",
+    "GlossDiv": {
+      "title": "S",
+      "GlossList": {
+        "GlossEntry": {
+          "ID": "SGML",
+          "SortAs": "SGML",
+          "GlossTerm": "Standard Generalized Markup Language",
+          "Acronym": "SGML",
+          "Abbrev": "ISO 8879:1986",
+          "GlossDef": {
+            "para": "A meta-markup language, used to create markup languages such as DocBook.",
+            "GlossSeeAlso": [
+              "GML",
+              "XML"
+            ]
+          },
+          "GlossSee": "markup"
+        }
+      }
+    }
+  }
+}
+"""
+
+print(parse_json(sample))
diff --git a/users/wpcarro/scratch/facebook/parsing/parser.py b/users/wpcarro/scratch/facebook/parsing/parser.py
new file mode 100644
index 0000000000..407bff61c9
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/parsing/parser.py
@@ -0,0 +1,28 @@
+class Parser(object):
+    def __init__(self, tokens):
+        self.tokens = tokens
+        self.i = 0
+
+    def prev(self):
+        return self.tokens[self.i - 1]
+
+    def curr(self):
+        return self.tokens[self.i]
+
+    def consume(self):
+        if not self.exhausted():
+            self.i += 1
+            return self.prev()
+
+    def match(self, xs):
+        if not self.exhausted() and self.curr() in xs:
+            self.consume()
+            return True
+        return False
+
+    def expect(self, xs):
+        if not self.match(xs):
+            raise Exception("Expected token \"{}\" but received \"{}\"".format(xs, self.curr()))
+
+    def exhausted(self):
+        return self.i >= len(self.tokens)
diff --git a/users/wpcarro/scratch/facebook/parsing/regex.py b/users/wpcarro/scratch/facebook/parsing/regex.py
new file mode 100644
index 0000000000..7fc2ef34e2
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/parsing/regex.py
@@ -0,0 +1,184 @@
+# Writing a small proof-of-concept...
+#   - lexer
+#   - parser
+#   - compiler
+# ...for regex.
+#
+# BNF
+# expression -> ( char_class | CHAR ) quantifier? ( "|" expression )*
+# char_class -> "[" CHAR+ "]"
+# quantifier -> "?" | "*" | "+" | "{" INT? "," INT? "}"
+#
+# Of the numerous things I do not support, here are a few items of which I'm
+# aware:
+#   - alternatives:   (a|b)
+#   - capture groups: (ab)cd
+
+from parser import Parser
+import string
+
+################################################################################
+# Top-Level API
+################################################################################
+
+def tokenize(xs):
+    """
+    Transform `xs` into a list of tokens.
+
+    Also: expand shorthand symbols using the following table:
+      - ? -> {0,1}
+      - * -> {0,}
+      - + -> {1,}
+    """
+    result = []
+    i = 0
+    shorthand = {
+        "?": ["{", 0, ",", 1, "}"],
+        "*": ["{", 0, ",", "}"],
+        "+": ["{", 1, ",", "}"],
+    }
+    while i < len(xs):
+        if xs[i] in shorthand:
+            for c in shorthand[xs[i]]:
+                result.append(c)
+            i += 1
+        elif xs[i] == "{":
+            result.append(xs[i])
+            i += 1
+            curr = ""
+            while xs[i] in string.digits:
+                curr += xs[i]
+                i += 1
+            result.append(int(curr))
+            assert xs[i] == ","
+            result.append(",")
+            i += 1
+            curr = ""
+            while xs[i] in string.digits:
+                curr += xs[i]
+                i += 1
+            result.append(int(curr))
+        else:
+            result.append(xs[i])
+            i += 1
+    return result
+
+def parse(expr):
+    """
+    Tokenize `expr` and convert it into a parse-tree.
+    """
+    tokens = tokenize(expr)
+    return parse_tokens(tokens)
+
+def compile(xs):
+    """
+    Transform `xs`, a parse-tree representing a regex, into a function that
+    accepts a string, and returns the substring that the regex matches.
+    """
+    def fn(input):
+        match = ""
+        i = 0
+        for x in xs:
+            matches, q = x[1], x[2]
+            lo, hi = q[1], q[2]
+            for j in range(lo):
+                if i < len(input) and input[i] in matches:
+                    match += input[i]
+                    i += 1
+                else:
+                    print("Failed to match {} with {}".format(input[i], matches))
+                    return None
+            if hi == float('inf'):
+                while i < len(input) and input[i] in matches:
+                    match += input[i]
+                    i += 1
+            else:
+                for j in range(hi - lo):
+                    if i < len(input) and input[i] in matches:
+                        match += input[i]
+                        i += 1
+        return match
+    return fn
+
+################################################################################
+# Helper Functions
+################################################################################
+
+def parse_tokens(tokens):
+    result = []
+    parser = Parser(tokens)
+    while not parser.exhausted():
+        result.append(parse_expression(parser))
+    return result
+
+def parse_expression(parser):
+    if parser.curr() == "[":
+        return parse_character_class(parser)
+    else:
+        return parse_character(parser)
+
+def parse_character_class(parser):
+    parser.expect("[")
+    beg = parser.consume()
+    parser.expect("-")
+    end = parser.consume()
+    parser.expect("]")
+    if parser.curr() == "{":
+        q = parse_quantifier(parser)
+    return char_class(xs=expand_range(beg, end), q=q)
+
+def parse_quantifier(parser):
+    parser.expect("{")
+    if parser.match([","]):
+        end = parser.consume()
+        parser.expect("}")
+        return quantifier(beg=0, end=end)
+    else:
+        beg = parser.consume()
+        parser.expect(",")
+        if parser.match(["}"]):
+            return quantifier(beg=beg)
+        else:
+            end = parser.consume()
+            parser.expect("}")
+            return quantifier(beg=beg, end=end)
+
+def parse_character(parser):
+    c = parser.consume()
+    q = None
+    if parser.curr() == "{":
+        q = parse_quantifier(parser)
+    return char_class(xs={c}, q=q)
+
+def char_class(xs=set(), q=None):
+    if not q:
+        q = quantifier(beg=1, end=1)
+    return ["CHARACTER_CLASS", xs, q]
+
+def expand_range(beg, end):
+    # TODO: Implement this
+    return {string.printable[i]
+            for i in range(string.printable.index(beg),
+                           string.printable.index(end) + 1)}
+
+def quantifier(beg=0, end=float('inf')):
+    return ['QUANTIFIER', beg, end]
+
+################################################################################
+# Tests
+################################################################################
+
+xs = [
+    ("[a-c]*[0-9]{2,3}", ["dog"]),
+    ("ca+t?", ["cat", "caaaat", "ca", "dog"]),
+]
+
+for re, inputs in xs:
+    print("Regex:  {}".format(re))
+    print("Tokens: {}".format(tokenize(re)))
+    print("Parsed: {}".format(parse(re)))
+    print("\nTESTS")
+    for input in inputs:
+        print("Attempting to match \"{}\"...".format(input))
+        parser = compile(parse(re))
+        print("Result: \"{}\"\n".format(parser(input)))
diff --git a/users/wpcarro/scratch/facebook/permutation-palindrome.py b/users/wpcarro/scratch/facebook/permutation-palindrome.py
new file mode 100644
index 0000000000..30603578ff
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/permutation-palindrome.py
@@ -0,0 +1,17 @@
+from collections import Counter
+
+def is_palindrome(x):
+    return len([count for _, count in Counter(x).items() if count % 2 == 1]) <= 1
+
+
+xs = [("civic", True),
+      ("ivicc", True),
+      ("civil", False),
+      ("livci", False)]
+
+for x, expected in xs:
+    result = is_palindrome(x)
+    print(x)
+    print(result)
+    assert result == expected
+    print("Success!")
diff --git a/users/wpcarro/scratch/facebook/polynomial-rolling-hash.py b/users/wpcarro/scratch/facebook/polynomial-rolling-hash.py
new file mode 100644
index 0000000000..0c7b7cb5a0
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/polynomial-rolling-hash.py
@@ -0,0 +1,72 @@
+def compute_hash(x):
+    """
+    Compute a unique fingerprint for the string input, `x`, as an integer using
+    the following equation:
+
+    x[0] * P^0 + x[1] * P^1 + ... x[n-1] * P^(n-1) % M
+
+    P and M are constants where P represents the next available prime number
+    that's GTE the number of unique characters you'll be hashing. In the case of
+    all lowercase characters, of which there are 26, the next available prime
+    number is 31.
+    """
+    p = 31
+    m = int(10e9) + 9 # large prime number
+    power = 0
+    result = 0
+    for c in x:
+        result += ord(c) * p**power
+        power += 1
+    return result % m
+
+class HashTable(object):
+    def __init__(self, size):
+        """
+        Create a hash table with `size` buckets.
+        """
+        buckets = []
+        for _ in range(size):
+            buckets.append([])
+        self.xs = buckets
+        self.compute_hash = lambda k: compute_hash(k) % size
+
+    def __repr__(self):
+        result = []
+        for bucket in self.xs:
+            for entry in bucket:
+                result.append(entry)
+        return "HashTable({})".format(",".join(str(x) for x in result))
+
+    def get(self, key):
+        """
+        Attempt to retrieve value stored under `key`.
+        """
+        h = self.compute_hash(key)
+        for k, v in self.xs[h]:
+            if k == key:
+                return v
+        return None
+
+    def put(self, key, val):
+        """
+        Set `key` to `val`; update value at `key` if it already exists.
+        """
+        h = self.compute_hash(key)
+        for i in range(len(self.xs[h])):
+            # Update entry if the key exists...
+            if self.xs[h][i][0] == key:
+                self.xs[h][i] = (key, val)
+                return None
+        # ...create a new entry otherwise
+        self.xs[h].append((key, val))
+
+    def delete(self, key):
+        """
+        Remove entry `key` from the hash table.
+        """
+        h = self.compute_hash(key)
+        for i in range(len(self.xs[h])):
+            k, v = self.xs[h][i]
+            if k == key:
+                self.xs[h].remove((k, v))
+                return
diff --git a/users/wpcarro/scratch/facebook/product-of-all-other-numbers.py b/users/wpcarro/scratch/facebook/product-of-all-other-numbers.py
new file mode 100644
index 0000000000..d381386b62
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/product-of-all-other-numbers.py
@@ -0,0 +1,33 @@
+from random import randint
+from math import floor
+
+# loop {forwards, backwards, up, down}
+# through a table of values, [[a]].
+
+def product(xs):
+    n = len(xs)
+    lhs = [1] * (n + 1)
+    for i in range(1, n):
+        lhs[i] = lhs[i - 1] * xs[i - 1]
+    rhs = [1] * (n + 1)
+    for i in range(n - 1, 0, -1):
+        rhs[i] = rhs[i + 1] * xs[i]
+    result = []
+    for i in range(n):
+        result.append(lhs[i] * rhs[i + 1])
+    return result
+
+def computed_expected(xs):
+    product = 1
+    for x in xs:
+        product *= x
+    return [floor(product / x) for x in xs]
+
+xs = [randint(1, 10) for _ in range(5)]
+expected = computed_expected(xs)
+result = product(xs)
+print(xs, result, expected)
+assert result == expected
+print("Success!")
+
+print(product([2, 4, 3, 10, 5]))
diff --git a/users/wpcarro/scratch/facebook/queue-two-stacks.py b/users/wpcarro/scratch/facebook/queue-two-stacks.py
new file mode 100644
index 0000000000..a71abeb005
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/queue-two-stacks.py
@@ -0,0 +1,20 @@
+from stack import Stack
+
+class Queue(object):
+    def __init__(self):
+        self.lhs = Stack()
+        self.rhs = Stack()
+
+    def enqueue(self, x):
+        self.rhs.push(x)
+
+    def dequeue(self, x):
+        y = self.rhs.pop()
+        while y:
+            self.lhs.push(y)
+            y = self.rhs.pop()
+        result = self.lhs.pop()
+        y = self.lhs.pop()
+        while y:
+            self.rhs.push(y)
+        return result
diff --git a/users/wpcarro/scratch/facebook/rabin-karp.py b/users/wpcarro/scratch/facebook/rabin-karp.py
new file mode 100644
index 0000000000..53a47b2783
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/rabin-karp.py
@@ -0,0 +1,27 @@
+def substring_exists(corpus, pattern):
+    """
+    Return True if `pattern` appears in `corpus`.
+
+    This function runs in O(m) time where n is equal to the length of
+    `corpus`. To improve the efficiency of this algorithm, use a hashing
+    function the reduces the number of collisions, which will consequently
+    reduce the number of string-to-string, linear comparisons.
+    """
+    m, n = len(corpus), len(pattern)
+    a = sum(ord(c) for c in corpus[0:n])
+    b = sum(ord(c) for c in pattern)
+
+    # (clumsily) prevent an off-by-one error...
+    if a == b and corpus[0:n] == pattern:
+        return True
+
+    for i in range(1, m - n):
+        # Update the hash of corpus by subtracting the hash of the character
+        # that is sliding out of view and adding the hash of the character that
+        # is sliding into view.
+        a = a - ord(corpus[i - 1]) + ord(corpus[i + n - 1])
+        # Integer comparison in O(0) time followed by string comparison in O(m)
+        # time.
+        if a == b and corpus[i:i + n] == pattern:
+            return True
+    return False
diff --git a/users/wpcarro/scratch/facebook/recursion-and-dynamic-programming/magic-index.py b/users/wpcarro/scratch/facebook/recursion-and-dynamic-programming/magic-index.py
new file mode 100644
index 0000000000..03b2de015d
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/recursion-and-dynamic-programming/magic-index.py
@@ -0,0 +1,33 @@
+from math import floor
+
+def find_magic_index_brute(xs):
+    for i in range(len(xs)):
+        if xs[i] == i:
+            return i
+    return -1
+
+def mid(lo, hi):
+    return lo + floor((hi - lo) / 2)
+
+def find_magic_index(xs):
+    lo, hi = 0, len(xs) - 1
+    return do_find_magic_index(xs, 0, len(xs) - 1)
+
+def do_find_magic_index(xs, lo, hi):
+    pass
+
+xss = [
+    [],
+    [-1,0,2,4,5,6],
+    [1,1,1,1,1,5],
+    [-2,-2,-2,-2,4],
+    [1,2,3,4,5],
+]
+
+for xs in xss:
+    print(xs)
+    a = find_magic_index_brute(xs)
+    b = find_magic_index(xs)
+    print(a, b)
+    assert a == b
+    print("Success!")
diff --git a/users/wpcarro/scratch/facebook/recursion-and-dynamic-programming/making-change.py b/users/wpcarro/scratch/facebook/recursion-and-dynamic-programming/making-change.py
new file mode 100644
index 0000000000..30c95a66c3
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/recursion-and-dynamic-programming/making-change.py
@@ -0,0 +1,56 @@
+# Given an infinite supply of:
+#   - quarters
+#   - dimes
+#   - nickels
+#   - pennies
+# Write a function to count the number of ways to make change of n.
+
+def get(table, row, col):
+    """
+    Defensively get cell `row`, `col` from `table`.
+    """
+    if row < 0 or row >= len(table):
+        return 0
+    if col < 0 or col >= len(table[0]):
+        return 0
+    return table[row][col]
+
+def print_table(table):
+    print('\n'.join([
+        ','.join([str(col) for col in table[row]])
+        for row in range(len(table))]))
+
+def init_table(rows=0, cols=0, default=0):
+    result = []
+    for row in range(rows):
+        r = []
+        for col in range(cols):
+            r.append(default)
+        result.append(r)
+    return result
+
+def make_change(n):
+    coins = [1, 5, 10, 25]
+    table = init_table(rows=len(coins), cols=n)
+
+    for row in range(len(table)):
+        for col in range(len(table[row])):
+            curr_coin = coins[row]
+            curr_n = col + 1
+            # a
+            a = get(table, row - 1, col)
+            # b
+            b = get(table, row, curr_n - curr_coin - 1)
+            # c
+            c = 1 if curr_coin <= curr_n else 0
+            # commit
+            if curr_coin == curr_n:
+                table[row][col] = a + c
+            else:
+                table[row][col] = a + b * c
+            # debug
+            print_table(table)
+            print()
+    return table[-1][-1]
+
+print(make_change(7))
diff --git a/users/wpcarro/scratch/facebook/recursion-and-dynamic-programming/paint-fill.py b/users/wpcarro/scratch/facebook/recursion-and-dynamic-programming/paint-fill.py
new file mode 100644
index 0000000000..e9e7f6a9c1
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/recursion-and-dynamic-programming/paint-fill.py
@@ -0,0 +1,36 @@
+from collection import deque
+
+def fill(point, canvas, color):
+    if x not in canvas:
+        return
+    elif y not in canvas[x]:
+        return
+
+    x, y = point
+    if canvas[y][x] == color:
+        return
+    canvas[y][x] = color
+    fill((x + 1, y), canvas, color)
+    fill((x - 1, y), canvas, color)
+    fill((x, y + 1), canvas, color)
+    fill((x, y - 1), canvas, color)
+
+def fill_bfs(point, canvas, color):
+    x, y = point
+    if x not in canvas:
+        return None
+    if y not in canvas[x]:
+        return None
+    xs = deque()
+    xs.append((x, y))
+    while xs:
+        x, y = xs.popleft()
+        canvas[y][x] = color
+        for x2, y2 in [(x - 1, y), (x + 1, y), (x, y - 1), (x, y + 1)]:
+            if x2 not in canvas:
+                continue
+            elif y2 not in canvas[x2]:
+                continue
+            if canvas[y2][x2] != color:
+                xs.append((x2, y2))
+    return None
diff --git a/users/wpcarro/scratch/facebook/recursion-and-dynamic-programming/parenthesize-bools.py b/users/wpcarro/scratch/facebook/recursion-and-dynamic-programming/parenthesize-bools.py
new file mode 100644
index 0000000000..f406d64e65
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/recursion-and-dynamic-programming/parenthesize-bools.py
@@ -0,0 +1,114 @@
+# BNF
+# expression -> bool ( ( '|' | '&' | '^' ) bool )*
+# bool       -> '0' | '1'
+
+def tokenize(xs):
+    result = []
+    for c in xs:
+        if c == '0':
+            result.append(0)
+        elif c == '1':
+            result.append(1)
+        elif c in "&|^":
+            result.append(c)
+        else:
+            raise Exception("Unexpected token, \"{}\"".format(c))
+    return result
+
+class Parser(object):
+    def __init__(self, tokens):
+        self.tokens = tokens
+        self.i = 0
+
+    def prev(self):
+        return self.tokens[self.i - 1]
+
+    def curr(self):
+        return self.tokens[self.i]
+
+    def match(self, xs):
+        if self.exhausted():
+            return False
+        if (self.curr() in xs):
+            self.consume()
+            return True
+        return False
+
+    def consume(self):
+        result = self.curr()
+        self.i += 1
+        return result
+
+    def exhausted(self):
+        return self.i >= len(self.tokens)
+
+def recursive_descent(tokens):
+    parser = Parser(tokens)
+    return parse_expression(parser)
+
+def parse_expression(parser):
+    lhs = parse_bool(parser)
+    while parser.match(['|', '&', '^']):
+        op = parser.prev()
+        rhs = parse_expression(parser)
+        lhs = [op, lhs, rhs]
+    return lhs
+
+def parse_bool(parser):
+    if parser.curr() == 0:
+        parser.consume()
+        return False
+    elif parser.curr() == 1:
+        parser.consume()
+        return True
+    else:
+        raise Exception("Unexpected token: {}".format(parser.curr()))
+
+def f(expr, result):
+    tokens = tokenize(expr)
+    tree = recursive_descent(tokens)
+    return do_f(tree, result)
+
+def do_f(tree, result):
+    if type(tree) == bool:
+        if tree == result:
+            return 1
+        else:
+            return 0
+
+    op, lhs, rhs = tree[0], tree[1], tree[2]
+    truth_tables = {
+        True: {
+            '|': [
+                (True, True),
+                (True, False),
+                (False, True),
+            ],
+            '&': [
+                (True, True),
+            ],
+            '^': [
+                (True, False),
+                (False, True),
+            ],
+        },
+        False: {
+            '|': [
+                (False, False),
+            ],
+            '&': [
+                (False, False),
+                (True, False),
+                (False, True),
+            ],
+            '^': [
+                (True, True),
+                (False, False),
+            ],
+        }
+    }
+
+    return sum([do_f(lhs, x) * do_f(rhs, y) for x, y in truth_tables[result][op]])
+
+print(f("1^0|0|1", False))
+print(f("1|0|1|1", False))
diff --git a/users/wpcarro/scratch/facebook/recursion-and-dynamic-programming/permutations.py b/users/wpcarro/scratch/facebook/recursion-and-dynamic-programming/permutations.py
new file mode 100644
index 0000000000..e23972d418
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/recursion-and-dynamic-programming/permutations.py
@@ -0,0 +1,13 @@
+def char_and_rest(i, xs):
+    return xs[i], xs[:i] + xs[i+1:]
+
+# perms :: String -> [String]
+def perms(xs):
+    if len(xs) == 1:
+        return [xs]
+    result = []
+    for c, rest in [char_and_rest(i, xs) for i in range(len(xs))]:
+        result += [c + perm for perm in perms(rest)]
+    return result
+
+print(perms("cat"))
diff --git a/users/wpcarro/scratch/facebook/recursion-and-dynamic-programming/robot-grid-traversal.py b/users/wpcarro/scratch/facebook/recursion-and-dynamic-programming/robot-grid-traversal.py
new file mode 100644
index 0000000000..9ccc08526a
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/recursion-and-dynamic-programming/robot-grid-traversal.py
@@ -0,0 +1,28 @@
+import random
+
+def factorial(n):
+    result = 1
+    for i in range(1, n + 1):
+        result *= i
+    return result
+
+def travel(a, b):
+    if a == b:
+        return 1
+
+    ax, ay = a
+    bx, by = b
+    if ax > bx or ay > by:
+        return 0
+
+    return sum([travel((ax + 1, ay), b), travel((ax, ay + 1), b)])
+
+def travel_compute(a, b):
+    bx, by = b
+    return int(factorial(bx + by) / (factorial(bx) * factorial(by)))
+
+a = (0, 0)
+b = (random.randint(1, 10), random.randint(1, 10))
+print("Travelling to {}, {}".format(b[0], b[1]))
+print(travel(a, b))
+print(travel_compute(a, b))
diff --git a/users/wpcarro/scratch/facebook/recursion-and-dynamic-programming/staircase.py b/users/wpcarro/scratch/facebook/recursion-and-dynamic-programming/staircase.py
new file mode 100644
index 0000000000..5eb4a85606
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/recursion-and-dynamic-programming/staircase.py
@@ -0,0 +1 @@
+# accidentally deleted my solution... TBI (again)
diff --git a/users/wpcarro/scratch/facebook/recursion-and-dynamic-programming/subsets.py b/users/wpcarro/scratch/facebook/recursion-and-dynamic-programming/subsets.py
new file mode 100644
index 0000000000..a6d26aa850
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/recursion-and-dynamic-programming/subsets.py
@@ -0,0 +1,41 @@
+# take-aways:
+#   - Use integers as lists of boolean values
+#   - Use 1 << n to compute 2^n where n = len(xs)
+
+def set_from_int(xs, n):
+    result = []
+    for i in range(len(xs)):
+        if n & (1 << i) != 0:
+            result.append(xs[i])
+    return result
+
+# subsets :: Set a -> List (Set a)
+def subsets(xs):
+    n = len(xs)
+    return [set_from_int(xs, i) for i in range(1 << n)]
+
+#   0 1 2
+# 0 N Y Y
+# 1 _ N Y
+# 2 _ _ N
+
+# For my interview, be able to compute *permutations* and *combinations*
+
+# This differs from permutations because this is about finding combinations...
+#
+# bottom-up
+# 0 =>        { }
+# 1 =>  {3}   {4}   {3}
+# 2 => {5,4} {5,3} {4,3}
+
+xs = [
+    ([], [[]]),
+    ([5], [[], [5]]),
+    ([5,4], [[],[5],[4],[5,4]]),
+]
+
+for x, expected in xs:
+    result = subsets(x)
+    print("subsets({}) => {} == {}".format(x, result, expected))
+    assert result == expected
+    print("Success!")
diff --git a/users/wpcarro/scratch/facebook/recursion-and-dynamic-programming/valid-parens.py b/users/wpcarro/scratch/facebook/recursion-and-dynamic-programming/valid-parens.py
new file mode 100644
index 0000000000..56f2c0b274
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/recursion-and-dynamic-programming/valid-parens.py
@@ -0,0 +1,50 @@
+def valid_parens(n):
+    if n == 0:
+        return []
+    if n == 1:
+        return ["()"]
+
+    result = set()
+    for x in valid_parens(n - 1):
+        result.add("({})".format(x))
+        result.add("(){}".format(x))
+        result.add("{}()".format(x))
+    return result
+
+def valid_parens_efficient(n):
+    result = []
+    curr = [''] * n**2
+    do_valid_parens_efficient(result, curr, 0, n, n)
+    return result
+
+def do_valid_parens_efficient(result, curr, i, lhs, rhs):
+    if lhs == 0 and rhs == 0:
+        result.append(''.join(curr))
+    else:
+        if lhs > 0:
+            curr[i] = '('
+            do_valid_parens_efficient(result, curr, i + 1, lhs - 1, rhs)
+        if rhs > lhs:
+            curr[i] = ')'
+            do_valid_parens_efficient(result, curr, i + 1, lhs, rhs - 1)
+
+# Avoids recursion by using either a stack or a queue. I think this version is
+# easier to understand.
+def valid_parens_efficient_2(n):
+    result = []
+    xs = []
+    xs.append(('', n, n))
+    while xs:
+        curr, lhs, rhs = xs.pop()
+        print(curr)
+        if lhs == 0 and rhs == 0:
+            result.append(''.join(curr))
+        if lhs > 0:
+            xs.append((curr + '(', lhs - 1, rhs))
+        if rhs > lhs:
+            xs.append((curr + ')', lhs, rhs - 1))
+    return result
+
+# print(valid_parens(4))
+print(valid_parens_efficient(3))
+print(valid_parens_efficient_2(3))
diff --git a/users/wpcarro/scratch/facebook/recursive-string-permutations.py b/users/wpcarro/scratch/facebook/recursive-string-permutations.py
new file mode 100644
index 0000000000..e4c61eff9f
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/recursive-string-permutations.py
@@ -0,0 +1,19 @@
+# permutations: no repeat characters
+
+def char_and_rest(i, xs):
+    return xs[i], xs[0:i] + xs[i + 1:]
+
+def permutations(xs):
+    if len(xs) == 1:
+        return [xs]
+    result = []
+    for c, rest in [char_and_rest(i, xs) for i in range(len(xs))]:
+        result += [c + perm for perm in permutations(rest)]
+    return result
+
+expected = ["cat", "cta", "act", "atc", "tca", "tac"]
+result = permutations("cat")
+print(result, expected)
+assert len(result) == len(expected)
+assert result == expected
+print("Success!")
diff --git a/users/wpcarro/scratch/facebook/reverse-linked-list.py b/users/wpcarro/scratch/facebook/reverse-linked-list.py
new file mode 100644
index 0000000000..820726733f
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/reverse-linked-list.py
@@ -0,0 +1,25 @@
+from linked_list import Node
+
+def reverse(node):
+    prev, curr, next = None, node, node.next
+
+    while curr:
+        curr.next = prev
+        prev = curr
+        curr = next
+        next = curr.next if curr else None
+    return prev
+
+one = Node(1)
+two = Node(2)
+three = Node(3)
+one.next = two
+two.next = three
+
+print(one)
+result = reverse(one)
+print(result)
+assert all([result == three,
+            three.next == two,
+            two.next == one])
+print("Success!")
diff --git a/users/wpcarro/scratch/facebook/reverse-string-in-place.py b/users/wpcarro/scratch/facebook/reverse-string-in-place.py
new file mode 100644
index 0000000000..72cd6c27a3
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/reverse-string-in-place.py
@@ -0,0 +1,14 @@
+# reverse :: [Char] -> ()
+def reverse(xs):
+    i = 0
+    j = len(xs) - 1
+    while i < j:
+        xs[i], xs[j] = xs[j], xs[i]
+        i += 1
+        j -= 1
+
+xs = [list("testing"), list("a"), list("to")]
+for x in xs:
+    print(x)
+    reverse(x)
+    print(x)
diff --git a/users/wpcarro/scratch/facebook/reverse-words.py b/users/wpcarro/scratch/facebook/reverse-words.py
new file mode 100644
index 0000000000..5a38b828a3
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/reverse-words.py
@@ -0,0 +1,8 @@
+# reverse_word :: [Char] -> ()
+def reverse_words(x):
+    pass
+
+x = list("This is a test")
+print(''.join(x))
+reverse_words(x)
+print(''.join(result))
diff --git a/users/wpcarro/scratch/facebook/scratch.py b/users/wpcarro/scratch/facebook/scratch.py
new file mode 100644
index 0000000000..e772d75847
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/scratch.py
@@ -0,0 +1,94 @@
+# This is a scratch pad for randomly selected questions
+
+# def char_and_rest(i, xs):
+#     return xs[i], xs[:i] + xs[i+1:]
+
+# def perms(xs):
+#     if len(xs) == 1:
+#         return [xs]
+#     result = []
+#     for i in range(len(xs)):
+#         c, rest = char_and_rest(i, xs)
+#         for perm in perms(rest):
+#             result.append(c + ''.join(perm))
+#     return result
+
+# print(perms(list("woah")))
+
+# def f(take_out, dine_in, served):
+#     j, k = 0, 0
+#     for i in range(len(served)):
+#         if j < len(take_out) and served[i] == take_out[j]:
+#             j += 1
+#         elif k < len(dine_in) and served[i] == dine_in[k]:
+#             k += 1
+#         else:
+#             return False
+#     if j < len(take_out) or k < len(dine_in):
+#         return False
+#     return True
+
+# take_out = [17, 8, 24]
+# dine_in = [12, 19, 2]
+# served = [17, 8, 12, 19, 24, 2]
+# print(f(take_out, dine_in, served))
+
+# def match(a, b):
+#     if a == '{':
+#         return b == '}'
+#     if a == '[':
+#         return b == ']'
+#     if a == '(':
+#         return b == ')'
+#     return False
+
+# def f(xs):
+#     s = []
+#     for c in xs:
+#         if c in {'{', '[', '('}:
+#             s.append(c)
+#         elif c in {'}', ']', ')'}:
+#             opener = s.pop()
+#             if not match(opener, c):
+#                 return False
+#     return len(s) == 0
+
+# assert f("{[]()}")
+# assert f("{[(])}") == False
+# assert f("{[}") == False
+# print("Success!")
+
+# def valid_bst(node):
+#     lhs = max_bst_value(node.left) if node.left else float('-inf')
+#     rhs = min_bst_value(node.right) if node.right else float('inf')
+
+#     return and([
+#         lhs <= node.value,
+#         rhs > node.value,
+#         valid_bst(node.left),
+#         valid_bst(node.right),
+#     ])
+
+import random
+import math
+
+def shuffle(xs):
+    n = len(xs)
+    for i in range(n - 1):
+        j = random.randint(i + 1, n - 1)
+        xs[i], xs[j] = xs[j], xs[i]
+    return xs
+
+def as_card(i):
+    if i not in range(1, 53):
+        raise Exception("Not a card")
+    # 1
+    suit = ['Hearts', 'Clubs', 'Diamonds', 'Spades'][math.floor((i - 1) / 13)]
+    n = ['Ace',2,3,4,5,6,7,8,9,10,'Jack','Queen','King'][(i - 1) % 13]
+    return '{} of {}'.format(n, suit)
+
+xs = list(range(1, 53))
+print(xs)
+shuffle(xs)
+for x in xs:
+    print(as_card(x))
diff --git a/users/wpcarro/scratch/facebook/second-largest-item-in-bst.py b/users/wpcarro/scratch/facebook/second-largest-item-in-bst.py
new file mode 100644
index 0000000000..2815dec9ee
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/second-largest-item-in-bst.py
@@ -0,0 +1,22 @@
+from collections import deque
+from node import Node, tree
+
+def find_largest(node):
+    while node.right:
+        node = node.right
+    return node.value
+
+def find_second_largest(node):
+    # parent of the rightmost, when rightmost is leaf
+    # max(rightmost.left)
+    prev = None
+    while node.right:
+        prev = node
+        node = node.right
+    if node.left:
+        return find_largest(node.left)
+    else:
+        return prev.value
+
+assert find_second_largest(tree) == 72
+print("Success!")
diff --git a/users/wpcarro/scratch/facebook/shuffle.py b/users/wpcarro/scratch/facebook/shuffle.py
new file mode 100644
index 0000000000..21a6a96c60
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/shuffle.py
@@ -0,0 +1,17 @@
+from random import randint
+
+def get_random(i, j):
+    return randint(i, j)
+
+def shuffle(xs):
+    for i in range(len(xs)):
+        j = get_random(i, len(xs) - 1)
+        xs[i], xs[j] = xs[j], xs[i]
+
+xs = list(range(1, 53))
+print(xs)
+assert len(set(xs)) == 52
+shuffle(xs)
+assert len(set(xs)) == 52
+print(xs)
+print("Success!")
diff --git a/users/wpcarro/scratch/facebook/stack.py b/users/wpcarro/scratch/facebook/stack.py
new file mode 100644
index 0000000000..2a843e2216
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/stack.py
@@ -0,0 +1,25 @@
+class Stack(object):
+    def __init__(self):
+        self.items = []
+
+    def __repr__(self):
+        return self.items.__repr__()
+
+    def push(self, x):
+        self.items.append(x)
+
+    def pop(self):
+        if not self.items:
+            return None
+        return self.items.pop()
+
+    def peek(self):
+        if not self.items:
+            return None
+        return self.items[-1]
+
+def from_list(xs):
+    result = Stack()
+    for x in xs:
+        result.push(x)
+    return result
diff --git a/users/wpcarro/scratch/facebook/stacking-boxes.py b/users/wpcarro/scratch/facebook/stacking-boxes.py
new file mode 100644
index 0000000000..7a3304bc51
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/stacking-boxes.py
@@ -0,0 +1,50 @@
+from random import randint
+
+class Box(object):
+    def __init__(self, w, h, d):
+        self.width  = w
+        self.depth  = d
+        self.height = h
+
+    def __repr__(self):
+        return "{}x{}x{}".format(self.width, self.depth, self.height)
+
+    def lt(self, b):
+        return all([
+            self.width  < b.width,
+            self.height < b.height,
+            self.depth  < b.depth,
+        ])
+
+    def gt(self, b):
+        return all([
+            self.width  > b.width,
+            self.height > b.height,
+            self.depth  > b.depth,
+        ])
+
+def random_box():
+    return Box(
+        randint(1, 10),
+        randint(1, 10),
+        randint(1, 10),
+    )
+
+xs = [random_box() for _ in range(5)]
+
+def highest_stack(xs, cache={}):
+    if not xs:
+        return 0
+    heights = []
+    for i in range(len(xs)):
+        x, rest = xs[i], xs[0:i] + xs[i+1:]
+        if cache and x in cache:
+            height = cache[x]
+        else:
+            height = x.height + highest_stack([b for b in rest if x.gt(b)], cache)
+            cache[x] = height
+        heights += [height]
+    return max(heights)
+
+print(xs)
+print(highest_stack(xs))
diff --git a/users/wpcarro/scratch/facebook/stock-price.py b/users/wpcarro/scratch/facebook/stock-price.py
new file mode 100644
index 0000000000..8e42f81523
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/stock-price.py
@@ -0,0 +1,16 @@
+def max_profit(xs):
+    buy = xs[0]
+    profit = xs[1] - xs[0]
+    for price in xs[1:]:
+        profit = max(profit, price - buy)
+        buy = min(buy, price)
+    return profit
+
+xs = [([10,7,5,8,11,9], 6),
+      ([10,8,7,6,5], -1)]
+
+for x, expected in xs:
+    result = max_profit(x)
+    print(x, result)
+    assert result == expected
+    print("Success!")
diff --git a/users/wpcarro/scratch/facebook/todo.org b/users/wpcarro/scratch/facebook/todo.org
new file mode 100644
index 0000000000..6ac99267db
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/todo.org
@@ -0,0 +1,60 @@
+* Array and string manipulation
+** DONE Merging Meeting Times
+** DONE Reverse String in Place
+** TODO Reverse Words
+** DONE Merge Sorted Arrays
+** DONE Cafe Order Checker
+* Hashing and hash tables
+** DONE Inflight Entertainment
+** DONE Permutation Palindrome
+** DONE Word Cloud Data
+** DONE Top Scores
+* Greedy Algorithms
+** DONE Apple Stocks
+** DONE Highest Product of 3
+** DONE Product of All Other Numbers
+** DONE Cafe Order Checker
+** DONE In-Place Shuffle
+* Sorting, searching, and logarithms
+** DONE Find Rotation Point
+** TODO Find Repeat, Space Edition
+** DONE Top Scores
+** DONE Merging Meeting Times
+* Trees and graphs
+** DONE Balanced Binary Tree
+** DONE Binary Search Tree Checker
+** DONE 2nd Largest Item in a Binary Search Tree
+** DONE Graph Coloring
+** DONE MeshMessage
+** DONE Find Repeat, Space Edition BEAST MODE
+* Dynamic programming and recursion
+** DONE Recursive String Permutations
+** DONE Compute nth Fibonacci Number
+** DONE Making Change
+** DONE The Cake Thief
+** DONE Balanced Binary Tree
+** DONE Binary Search Tree Checker
+** DONE 2nd Largest Item in a Binary Search Tree
+* Queues and stacks
+** DONE Largest Stack
+** DONE Implement A Queue With Two Stacks
+** DONE Parenthesis Matching
+** DONE Bracket Validator
+* Linked lists
+** DONE Delete Node
+** DONE Does This Linked List Have A Cycle?
+** DONE Reverse A Linked List
+** DONE Kth to Last Node in a Singly-Linked List
+** DONE Find Repeat, Space Edition BEAST MODE
+* General programming
+** TODO Rectangular Love
+** TODO Temperature Tracker
+* Bit manipulation
+** DONE The Stolen Breakfast Drone
+* Combinatorics, probability, and other math
+** TODO Which Appears Twice
+** TODO Find in Ordered Set
+** TODO In-Place Shuffle
+** TODO Simulate 5-sided die
+** TODO Simulate 7-sided die
+** TODO Two Egg Problem
diff --git a/users/wpcarro/scratch/facebook/top-scores.py b/users/wpcarro/scratch/facebook/top-scores.py
new file mode 100644
index 0000000000..c8a10ae5f1
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/top-scores.py
@@ -0,0 +1,20 @@
+import random
+from collections import deque
+
+def sorted(xs):
+    result = [0] * 100
+    for x in xs:
+        result[x - 1] += 1
+
+    answer = deque()
+    for i in range(len(result)):
+        x = result[i]
+        for _ in range(x):
+            answer.appendleft(i + 1)
+
+    return list(answer)
+
+scores = [random.choice(range(70, 100)) for _ in range(20)]
+print(scores)
+result = sorted(scores)
+print(result)
diff --git a/users/wpcarro/scratch/facebook/topo-sort.py b/users/wpcarro/scratch/facebook/topo-sort.py
new file mode 100644
index 0000000000..874005a019
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/topo-sort.py
@@ -0,0 +1,61 @@
+import random
+from heapq import heappush, heappop
+from collections import deque
+
+# A topological sort returns the vertices of a graph sorted in an ascending
+# order by the number of incoming edges each vertex has.
+#
+# A few algorithms for solving this exist, and at the time of this writing, I
+# know none. I'm going to focus on two:
+#   1. Kahn's
+#   2. DFS (TODO)
+
+def count_in_edges(graph):
+    result = {k: 0 for k in graph.keys()}
+    for xs in graph.values():
+        for x in xs:
+            result[x] += 1
+    return result
+
+# Kahn's algorithm for returning a topological sorting of the vertices in
+# `graph`.
+def kahns_sort(graph):
+    result = []
+    q = deque()
+    in_edges = count_in_edges(graph)
+    for x in [k for k, v in in_edges.items() if v == 0]:
+        q.append(x)
+    while q:
+        x = q.popleft()
+        result.append(x)
+        for c in graph[x]:
+            in_edges[c] -= 1
+            if in_edges[c] == 0:
+                q.append(c)
+    return result
+
+graphs = [
+    {
+        0: [],
+        1: [],
+        2: [3],
+        3: [1],
+        4: [0, 1],
+        5: [0, 2],
+    },
+    {
+        'A': ['C', 'D'],
+        'B': ['D', 'E'],
+        'C': [],
+        'D': ['F', 'G'],
+        'E': [],
+        'F': [],
+        'G': ['I'],
+        'H': ['I'],
+        'I': [],
+    }
+]
+
+print("--- Kahn's --- ")
+for graph in graphs:
+    print(kahns_sort(graph))
diff --git a/users/wpcarro/scratch/facebook/traversals.py b/users/wpcarro/scratch/facebook/traversals.py
new file mode 100644
index 0000000000..e2565a3231
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/traversals.py
@@ -0,0 +1,100 @@
+from math import floor
+
+# Lists
+def cycle_backwards(times, xs):
+    n = len(xs)
+    for i in range(n * times):
+        print(xs[n - 1 - i % n])
+
+def cycle_forwards(times, xs):
+    n = len(xs)
+    for i in range(n * times):
+        print(xs[i % n])
+
+def backwards(xs):
+    n = len(xs)
+    for i in range(n):
+        print(xs[n - 1 - i])
+
+def forwards(xs):
+    for i in range(len(xs)):
+        print(xs[i])
+
+xs = [2, 5, 6, 9, 12]
+
+print("Forwards")
+forwards(xs)
+print("Backwards")
+backwards(xs)
+print("Cycle forwards")
+cycle_forwards(2, xs)
+print("Cycle backwards")
+cycle_backwards(2, xs)
+
+# Tables
+def tblr(table):
+    for row in range(len(table)):
+        for col in range(len(table[row])):
+            print(table[row][col])
+
+def tbrl(table):
+    for row in range(len(table)):
+        n = len(table[row])
+        for col in range(n):
+            print(table[row][n - 1 - col])
+
+def btlr(table):
+    n = len(table)
+    for row in range(n):
+        for col in range(len(table[row])):
+            print(table[n - 1 - row][col])
+
+def btrl(table):
+    rows = len(table)
+    for row in range(rows):
+        cols = len(table[row])
+        for col in range(cols):
+            print(table[rows - 1 - row][cols - 1 - col])
+
+def special(table):
+    rows = len(table)
+    cols = len(table[0])
+    for col in range(cols):
+        for row in range(rows):
+            print(table[row][col])
+
+def double_bonus(table):
+    rows = len(table)
+    cols = len(table[0])
+    for i in range(rows):
+        row = i
+        for col in range(cols):
+            print(table[row][col % cols])
+            row = (row + 1) % rows
+
+def free(table):
+    rows = len(table)
+    cols = len(table[0])
+    d = rows * cols
+    for i in range(d):
+        row = floor((i % d) / cols)
+        col = i % cols
+        print(table[row][col])
+
+table = [[1,2,3,4],
+         [5,6,7,8]]
+
+print("Top->Bottom, Left->Right")
+tblr(table)
+print("Top->Bottom, Right->Left")
+tbrl(table)
+print("Bottom->Top, Left->Right")
+btlr(table)
+print("Bottom->Top, Right->Left")
+btrl(table)
+print("Special")
+special(table)
+print("2x Bonus")
+double_bonus(table)
+print("Free")
+free(table)
diff --git a/users/wpcarro/scratch/facebook/utils.py b/users/wpcarro/scratch/facebook/utils.py
new file mode 100644
index 0000000000..9a3e8a045e
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/utils.py
@@ -0,0 +1,19 @@
+def init_table(rows=0, cols=0, default=None):
+    table = []
+    for row in range(rows):
+        x = []
+        for col in range(cols):
+            x.append(default)
+        table.append(x)
+    return table
+
+def get(table, row, col, default=0):
+    if row < 0 or col < 0:
+        return default
+    return table[row][col]
+
+def print_table(table):
+    result = []
+    for row in range(len(table)):
+        result.append(' '.join([str(cell) for cell in table[row]]))
+    print('\n'.join(result))
diff --git a/users/wpcarro/scratch/facebook/word-cloud.py b/users/wpcarro/scratch/facebook/word-cloud.py
new file mode 100644
index 0000000000..88422e3631
--- /dev/null
+++ b/users/wpcarro/scratch/facebook/word-cloud.py
@@ -0,0 +1,32 @@
+def normalize(x):
+    noise = ".,;-"
+    for y in noise:
+        if x.endswith(y):
+            return normalize(x[0:-1])
+        if x.startswith(y):
+            return normalize(x[1:])
+    return x.lower()
+
+def word_cloud(xs):
+    result = dict()
+
+    for x in xs.split(' '):
+        k = normalize(x)
+        if k in result:
+            result[k] += 1
+        else:
+            result[k] = 1
+
+    return result
+
+result = word_cloud("This is just the beginning. The UK will lockdown again.")
+assert result.get('this') == 1
+assert result.get('is') == 1
+assert result.get('just') == 1
+assert result.get('the') == 2
+assert result.get('beginning') == 1
+assert result.get('uk') == 1
+assert result.get('will') == 1
+assert result.get('lockdown') == 1
+assert result.get('again') == 1
+print("Success!")
diff --git a/users/wpcarro/scratch/groceries/.envrc b/users/wpcarro/scratch/groceries/.envrc
new file mode 100644
index 0000000000..a4a62da526
--- /dev/null
+++ b/users/wpcarro/scratch/groceries/.envrc
@@ -0,0 +1,2 @@
+source_up
+use_nix
diff --git a/users/wpcarro/scratch/groceries/export.hs b/users/wpcarro/scratch/groceries/export.hs
new file mode 100644
index 0000000000..ed43c9a3e8
--- /dev/null
+++ b/users/wpcarro/scratch/groceries/export.hs
@@ -0,0 +1,22 @@
+module Main where
+
+import qualified Data.List as L
+
+(|>) :: a -> (a -> b) -> b
+x |> f = f x
+
+-- | Ignore items with zero quantity (i.e. "0x") and comments (i.e. "#")
+isUndesirableOutput :: String -> Bool
+isUndesirableOutput x =
+  (L.isPrefixOf "- 0x" x) || (L.isPrefixOf "#" x)
+
+-- | Run this to export the grocery list.
+main :: IO ()
+main = do
+  content <- readFile "./list.org"
+  content
+    |> lines
+    |> filter (not . isUndesirableOutput)
+    |> unlines
+    |> putStrLn
+  pure ()
diff --git a/users/wpcarro/scratch/groceries/list.org b/users/wpcarro/scratch/groceries/list.org
new file mode 100644
index 0000000000..a823b2a8eb
--- /dev/null
+++ b/users/wpcarro/scratch/groceries/list.org
@@ -0,0 +1,112 @@
+# The sections are sorted such that the first section is likely the first area
+# in the grocery store you'll encounter.
+#
+# This version is written for Tesco Metro in London Bridge.
+* Beer
+- 0x beer (6x)
+* Bread
+- 0x GF bread
+- 0x flour
+- 0x GF flour
+* Produce
+- 0x brocoli
+- 0x green beans
+- 0x green asparagus
+- 2x spinach greens
+- 0x romaine lettuce head
+- 0x tomatoes
+- 0x zucchini
+- 0x lemons
+- 1x limes
+- 0x large carrot
+- 2x garlic
+- 1x green onions
+- 0x onions
+- 0x avocado
+- 0x basil plant
+- 0x jalapeno
+- 0x red pepper
+- 0x green pepper
+- 0x cherry tomatoes
+- 0x potato
+- 0x bag dry black beans
+- 1x Scotch Bonnet pepper
+* Spices
+- 0x onion powder
+- 0x garlic powder
+- 0x chicken bouillon
+- 0x oregano
+- 0x red pepper flakes
+- 0x basil plant
+- 0x cilantro plant
+* Meat
+- 0x sausages
+- 0x steak
+- 0x chicken breasts
+- 0x chicken legs
+- 0x lamb
+- 0x ground beef
+* Frozen
+- 0x Salmon
+- 0x white fish
+- 0x shrimp
+- 0x bag green beans
+- 1x bag peas
+- 0x bag corn
+* Dairy
+- 1x unsalted butter
+- 0x coconut milk
+- 2x egg cartons (12x each)
+- 2x sour cream
+- 0x cheddar cheese
+- 0x parmesan
+- 0x gouda
+- 0x random cheese
+* Pasta
+- 0x box of quinoa
+- 0x box of rice
+- 1x GF pasta
+- 0x tortellini / ravioli
+- 0x tomato sauce
+- 0x tomato paste
+- 0x can diced tomatoes
+- 0x pesto
+* Oil
+- 0x olive oil
+- 0x sesame oil
+- 0x avocado oil
+- 0x coconut oil
+- 0x white wine vinegar
+* Condiments
+- 0x red Tabasco
+- 0x green Tabasco
+- 0x habanero Tabasco
+- 0x BBQ sauce
+- 0x french mustard
+- 0x ketchup
+- 0x oyster sauce
+- 0x soy sauce
+- 0x Srirachi sauce
+* Nuts
+- 0x almonds
+- 0x walnuts
+- 0x peanuts
+- 0x cashews
+- 0x Brazil nuts
+- 0x mixed nuts
+- 0x peanuts
+- 2x peanut butter
+* Sugar
+- 0x Lindt chocolate
+* Asian
+- 0x red curry
+- 0x green curry
+- 0x coconut cream
+* Wine
+- 0x red wine
+- 0x white wine
+* Miscellaneous
+- 0x coffee beans
+- 0x tea
+- 0x AA batteries
+- 0x rubbing alcohol
diff --git a/users/wpcarro/scratch/groceries/shell.nix b/users/wpcarro/scratch/groceries/shell.nix
new file mode 100644
index 0000000000..4d5b412a08
--- /dev/null
+++ b/users/wpcarro/scratch/groceries/shell.nix
@@ -0,0 +1,5 @@
+let
+  briefcase = import <briefcase> {};
+in briefcase.buildHaskell.shell {
+  deps = hpkgs: [];
+}
diff --git a/users/wpcarro/scratch/haskell-programming-from-first-principles/.envrc b/users/wpcarro/scratch/haskell-programming-from-first-principles/.envrc
new file mode 100644
index 0000000000..a4a62da526
--- /dev/null
+++ b/users/wpcarro/scratch/haskell-programming-from-first-principles/.envrc
@@ -0,0 +1,2 @@
+source_up
+use_nix
diff --git a/users/wpcarro/scratch/haskell-programming-from-first-principles/.ghci b/users/wpcarro/scratch/haskell-programming-from-first-principles/.ghci
new file mode 100644
index 0000000000..12aab7f08e
--- /dev/null
+++ b/users/wpcarro/scratch/haskell-programming-from-first-principles/.ghci
@@ -0,0 +1 @@
+:set prompt "> "
diff --git a/users/wpcarro/scratch/haskell-programming-from-first-principles/applicative.hs b/users/wpcarro/scratch/haskell-programming-from-first-principles/applicative.hs
new file mode 100644
index 0000000000..8259606da3
--- /dev/null
+++ b/users/wpcarro/scratch/haskell-programming-from-first-principles/applicative.hs
@@ -0,0 +1,213 @@
+module ApplicativeScratch where
+
+import Data.Function ((&))
+
+import Control.Applicative (liftA3)
+import qualified Data.List as List
+import qualified GHC.Base as Base
+
+--------------------------------------------------------------------------------
+
+-- xs :: [(Integer, Integer)]
+-- xs = zip [1..3] [4..6]
+
+-- added :: Maybe Integer
+-- added =
+--   (+3) <$> (lookup 3 xs)
+
+--------------------------------------------------------------------------------
+
+-- y :: Maybe Integer
+-- y = lookup 3 xs
+
+-- z :: Maybe Integer
+-- z = lookup 2 xs
+
+-- tupled :: Maybe (Integer, Integer)
+-- tupled = Base.liftA2 (,) y z
+
+--------------------------------------------------------------------------------
+
+-- x :: Maybe Int
+-- x = List.elemIndex 3 [1..5]
+
+-- y :: Maybe Int
+-- y = List.elemIndex 4 [1..5]
+
+-- maxed :: Maybe Int
+-- maxed = Base.liftA2 max x y
+
+--------------------------------------------------------------------------------
+
+xs = [1..3]
+ys = [4..6]
+
+x :: Maybe Integer
+x = lookup 3 $ zip xs ys
+
+y :: Maybe Integer
+y = lookup 2 $ zip xs ys
+
+summed :: Maybe Integer
+summed = sum <$> Base.liftA2 (,) x y
+
+--------------------------------------------------------------------------------
+
+newtype Identity a = Identity a deriving (Eq, Show)
+
+instance Functor Identity where
+  fmap f (Identity x) = Identity (f x)
+
+instance Applicative Identity where
+  pure = Identity
+  (Identity f) <*> (Identity x) = Identity (f x)
+
+--------------------------------------------------------------------------------
+
+newtype Constant a b =
+  Constant { getConstant :: a }
+  deriving (Eq, Ord, Show)
+
+instance Functor (Constant a) where
+  fmap _ (Constant x) = Constant x
+
+instance Monoid a => Applicative (Constant a) where
+  pure _ = Constant mempty
+  (Constant x) <*> (Constant y) = Constant (x <> y)
+
+--------------------------------------------------------------------------------
+
+one = const <$> Just "Hello" <*> Just "World"
+
+two :: Maybe (Integer, Integer, String, [Integer])
+two = (,,,) <$> (Just 90)
+            <*> (Just 10)
+            <*> (Just "Tierness")
+            <*> (Just [1..3])
+
+--------------------------------------------------------------------------------
+
+data List a = Nil | Cons a (List a) deriving (Eq, Show)
+
+instance Semigroup (List a) where
+  Nil <> xs = xs
+  xs <> Nil = xs
+  (Cons x xs) <> ys = Cons x (xs <> ys)
+
+instance Functor List where
+  fmap f Nil = Nil
+  fmap f (Cons x xs) = Cons (f x) (fmap f xs)
+
+instance Applicative List where
+  pure x = Cons x Nil
+  Nil <*> _ = Nil
+  _ <*> Nil = Nil
+  (Cons f fs) <*> xs =
+    (f <$> xs) <> (fs <*> xs)
+
+toList :: List a -> [a]
+toList Nil = []
+toList (Cons x xs) = x : toList xs
+
+fromList :: [a] -> List a
+fromList [] = Nil
+fromList (x:xs) = Cons x (fromList xs)
+
+--------------------------------------------------------------------------------
+
+newtype ZipList' a =
+  ZipList' [a]
+  deriving (Eq, Show)
+
+-- instance Eq a => EqProp (ZipList' a) where
+--   (ZipList' lhs) =-= (ZipList' rhs) =
+--     (take 1000 lhs) `eq` (take 1000 rhs)
+
+instance Functor ZipList' where
+  fmap f (ZipList' xs) = ZipList' $ fmap f xs
+
+instance Applicative ZipList' where
+  pure x = ZipList' (repeat x)
+  (ZipList' fs) <*> (ZipList' xs) =
+    ZipList' $ zipWith ($) fs xs
+
+--------------------------------------------------------------------------------
+
+data Validation e a
+  = Failure e
+  | Success a
+  deriving (Eq, Show)
+
+instance Functor (Validation e) where
+  fmap f (Failure x) = Failure x
+  fmap f (Success x) = Success (f x)
+
+instance Monoid e => Applicative (Validation e) where
+  pure = undefined
+  (Success f) <*> (Success x) = Success (f x)
+  _ <*> (Failure x) = Failure x
+  (Failure x) <*> _ = Failure x
+
+data Error
+  = DivideByZero
+  | StackOverflow
+  deriving (Eq, Show)
+
+--------------------------------------------------------------------------------
+
+stops :: String
+stops = "pbtdkg"
+
+vowels :: String
+vowels = "aeiou"
+
+combos :: [a] -> [b] -> [c] -> [(a, b, c)]
+combos xs ys zs =
+  liftA3 (,,) xs ys zs
+
+--------------------------------------------------------------------------------
+
+data Pair a = Pair a a deriving Show
+
+instance Functor Pair where
+  fmap f (Pair x y) = Pair (f x) (f y)
+
+instance Applicative Pair where
+  pure x = Pair x x
+  (Pair f g) <*> (Pair x y) = Pair (f x) (g x)
+
+p :: Pair Integer
+p = Pair 1 2
+
+--------------------------------------------------------------------------------
+
+data Two a b = Two a b
+
+instance Functor (Two a) where
+  fmap f (Two x y) = Two x (f y)
+
+instance Monoid a => Applicative (Two a) where
+  pure x = Two mempty x
+  _ <*> _ = undefined
+
+--------------------------------------------------------------------------------
+
+data Three a b c = Three a b c
+
+instance Functor (Three a b) where
+  fmap f (Three x y z) = Three x y (f z)
+
+instance (Monoid a, Monoid b) => Applicative (Three a b) where
+  pure x = Three mempty mempty x
+  (Three a b f) <*> (Three x y z) = Three (a <> x) (b <> y) (f z)
+
+--------------------------------------------------------------------------------
+
+data Three' a b = Three' a b b
+
+instance Functor (Three' a) where
+  fmap f (Three' x y z) = Three' x (f y) (f z)
+
+instance Monoid a => Applicative (Three' a) where
+  pure x = Three' mempty x x
+  (Three' a f g) <*> (Three' x y z) = Three' (a <> x) (f y) (g z)
diff --git a/users/wpcarro/scratch/haskell-programming-from-first-principles/basic-libraries.hs b/users/wpcarro/scratch/haskell-programming-from-first-principles/basic-libraries.hs
new file mode 100644
index 0000000000..bb1f89987e
--- /dev/null
+++ b/users/wpcarro/scratch/haskell-programming-from-first-principles/basic-libraries.hs
@@ -0,0 +1,60 @@
+module BasicLibrariesScratch where
+
+import Data.Function ((&))
+
+--------------------------------------------------------------------------------
+newtype DList a = DL { unDL :: [a] -> [a] }
+
+instance (Show a) => Show (DList a) where
+  show (DL x) = "DL " ++ show (x [])
+
+-- | Create an empty difference list.
+emptyDList :: DList a
+emptyDList = DL $ \xs -> xs
+{-# INLINE emptyDList #-}
+
+-- | Create a difference list with `x` as the only member.
+singleton :: a -> DList a
+singleton x =  DL $ \xs -> x : xs
+{-# INLINE singleton #-}
+
+-- | Convert the DList into a list.
+toList :: DList a -> [a]
+toList (DL unDL) = unDL mempty
+{-# INLINE toList #-}
+
+-- | Add an element to the end of a DList.
+infixr `snoc`
+snoc :: a -> DList a -> DList a
+snoc x (DL xs) = DL $ \ys -> xs (x : ys)
+{-# INLINE snoc #-}
+
+-- | Add an element to the beginning of a DList.
+infixr `cons`
+cons :: a -> DList a -> DList a
+cons x (DL xs) = DL $ \ys -> x : xs ys
+{-# INLINE cons #-}
+
+-- | Combine two DLists together.
+append :: DList a -> DList a -> DList a
+append (DL xs) (DL ys) = DL $ \zs -> zs & ys & xs
+{-# INLINE append #-}
+
+--------------------------------------------------------------------------------
+data Queue a =
+  Queue { one :: [a]
+        , two :: [a]
+        } deriving (Show, Eq)
+
+emptyQueue :: Queue a
+emptyQueue = Queue mempty mempty
+
+enqueue :: a -> Queue a -> Queue a
+enqueue x (Queue en de) = Queue (x:en) de
+
+dequeue :: Queue a -> Maybe (a, Queue a)
+dequeue (Queue [] []) = Nothing
+dequeue (Queue en []) =
+  let (d:de) = reverse en
+  in Just (d, Queue de [])
+dequeue (Queue en (d:de)) = Just (d, Queue en de)
diff --git a/users/wpcarro/scratch/haskell-programming-from-first-principles/composing-types.hs b/users/wpcarro/scratch/haskell-programming-from-first-principles/composing-types.hs
new file mode 100644
index 0000000000..378cfb7cea
--- /dev/null
+++ b/users/wpcarro/scratch/haskell-programming-from-first-principles/composing-types.hs
@@ -0,0 +1,75 @@
+module ComposingTypesScratch where
+
+import Data.Function ((&))
+import Data.Bifunctor
+
+import qualified Data.Foldable as F
+
+--------------------------------------------------------------------------------
+
+newtype Identity a =
+  Identity { getIdentity :: a }
+  deriving (Eq, Show)
+
+newtype Compose f g a =
+  Compose { getCompose :: f (g a) }
+  deriving (Eq, Show)
+
+--------------------------------------------------------------------------------
+
+instance (Functor f, Functor g) => Functor (Compose f g) where
+  fmap f (Compose getCompose) = Compose $ (fmap . fmap) f getCompose
+
+instance (Applicative f, Applicative g) => Applicative (Compose f g) where
+  pure x = x & pure & pure & Compose
+  fgf <*> fga = undefined
+
+--------------------------------------------------------------------------------
+
+instance (Foldable f, Foldable g) => Foldable (Compose f g) where
+  foldMap toMonoid x = undefined
+
+instance (Traversable f, Traversable g) => Traversable (Compose f g) where
+  traverse = undefined
+
+--------------------------------------------------------------------------------
+
+data Deux a b = Deux a b deriving (Show, Eq)
+
+instance Bifunctor Deux where
+  bimap f g (Deux x y) = Deux (f x) (g y)
+
+data Const a b = Const a deriving (Show, Eq)
+
+instance Bifunctor Const where
+  bimap f _ (Const x) = Const (f x)
+
+data Drei a b c = Drei a b c deriving (Show, Eq)
+
+instance Bifunctor (Drei a) where
+  bimap f g (Drei x y z) = Drei x (f y) (g z)
+
+data SuperDrei a b c = SuperDrei a b deriving (Show, Eq)
+
+instance Bifunctor (SuperDrei a) where
+  bimap f g (SuperDrei x y) = SuperDrei x (f y)
+
+data SemiDrei a b c = SemiDrei a deriving (Show, Eq)
+
+instance Bifunctor (SemiDrei a) where
+  bimap _ _ (SemiDrei x) = SemiDrei x
+
+data Quadriceps a b c d = Quadzzz a b c d
+
+instance Bifunctor (Quadriceps a b) where
+  bimap f g (Quadzzz w x y z) = Quadzzz w x (f y) (g z)
+
+-- | Analogue for Either
+data LeftRight a b
+  = Failure a
+  | Success b
+  deriving (Show, Eq)
+
+instance Bifunctor LeftRight where
+  bimap f _ (Failure x) = Failure (f x)
+  bimap _ g (Success y) = Success (g y)
diff --git a/users/wpcarro/scratch/haskell-programming-from-first-principles/foldable.hs b/users/wpcarro/scratch/haskell-programming-from-first-principles/foldable.hs
new file mode 100644
index 0000000000..5b59d9e9ba
--- /dev/null
+++ b/users/wpcarro/scratch/haskell-programming-from-first-principles/foldable.hs
@@ -0,0 +1,107 @@
+module FoldableScratch where
+
+import Data.Function ((&))
+
+--------------------------------------------------------------------------------
+
+sum :: (Foldable t, Num a) => t a -> a
+sum xs =
+  foldr (+) 0 xs
+
+product :: (Foldable t, Num a) => t a -> a
+product xs =
+  foldr (*) 1 xs
+
+elem :: (Foldable t, Eq a) => a -> t a -> Bool
+elem y xs =
+  foldr (\x acc -> if acc then acc else y == x) False xs
+
+minimum :: (Foldable t, Ord a) => t a -> Maybe a
+minimum xs =
+  foldr (\x acc ->
+           case acc of
+             Nothing   -> Just x
+             Just curr -> Just (min curr x)) Nothing xs
+
+maximum :: (Foldable t, Ord a) => t a -> Maybe a
+maximum xs =
+  foldr (\x acc ->
+           case acc of
+             Nothing   -> Nothing
+             Just curr -> Just (max curr x)) Nothing xs
+
+-- TODO: How could I use QuickCheck to see if Prelude.null and this null return
+-- the same results for the same inputs?
+null :: (Foldable t) => t a -> Bool
+null xs =
+  foldr (\_ _ -> False) True xs
+
+length :: (Foldable t) => t a -> Int
+length xs =
+  foldr (\_ acc -> acc + 1) 0 xs
+
+toList :: (Foldable t) => t a -> [a]
+toList xs =
+  reverse $ foldr (\x acc -> x : acc) [] xs
+
+fold :: (Foldable t, Monoid m) => t m -> m
+fold xs =
+  foldr mappend mempty xs
+
+foldMap :: (Foldable t, Monoid m) => (a -> m) -> t a -> m
+foldMap f xs =
+  foldr (\x acc -> mappend (f x) acc) mempty xs
+
+--------------------------------------------------------------------------------
+
+data List a = Nil | Cons a (List a) deriving (Eq, Show)
+
+instance Foldable List where
+  foldr f acc (Cons x rest) = foldr f (f x acc) rest
+  foldr f acc Nil = acc
+
+fromList :: [a] -> List a
+fromList [] = Nil
+fromList (x:rest) = Cons x (fromList rest)
+
+--------------------------------------------------------------------------------
+
+data Constant a b = Constant b deriving (Eq, Show)
+
+-- TODO: Is this correct?
+instance Foldable (Constant a) where
+  foldr f acc (Constant x) = f x acc
+
+--------------------------------------------------------------------------------
+
+data Two a b = Two a b deriving (Eq, Show)
+
+instance Foldable (Two a) where
+  foldr f acc (Two x y) = f y acc
+
+--------------------------------------------------------------------------------
+
+data Three a b c = Three a b c deriving (Eq, Show)
+
+instance Foldable (Three a b) where
+  foldr f acc (Three x y z) = f z acc
+
+--------------------------------------------------------------------------------
+
+data Three' a b = Three' a b b deriving (Eq, Show)
+
+instance Foldable (Three' a) where
+  foldr f acc (Three' x y z) = acc & f z & f y
+
+--------------------------------------------------------------------------------
+
+data Four' a b = Four' a b b b deriving (Eq, Show)
+
+instance Foldable (Four' a) where
+  foldr f acc (Four' w x y z) = acc & f z & f y & f x
+
+--------------------------------------------------------------------------------
+
+filterF :: (Applicative f, Foldable t, Monoid (f a)) => (a -> Bool) -> t a -> f a
+filterF pred xs =
+  foldr (\x acc -> if pred x then pure x `mappend` acc else acc) mempty xs
diff --git a/users/wpcarro/scratch/haskell-programming-from-first-principles/io.hs b/users/wpcarro/scratch/haskell-programming-from-first-principles/io.hs
new file mode 100644
index 0000000000..1de8937fce
--- /dev/null
+++ b/users/wpcarro/scratch/haskell-programming-from-first-principles/io.hs
@@ -0,0 +1,35 @@
+module IOScratch where
+
+import qualified System.Environment as SE
+import qualified System.IO as SIO
+--------------------------------------------------------------------------------
+
+docs :: String
+docs = "Pass -e to encrypt and -d to decrypt."
+
+encryptStdin :: IO ()
+encryptStdin = do
+  char <- SIO.hGetChar SIO.stdin
+  -- encrypt char
+  SIO.hPutStr SIO.stdout [char]
+
+decryptStdin :: IO ()
+decryptStdin = do
+  char <- SIO.hGetChar SIO.stdin
+  -- decrypt char
+  SIO.hPutStr SIO.stdout [char]
+
+main :: IO ()
+main = do
+  args <- SE.getArgs
+  case args of
+    [] ->
+      putStrLn $ "You did not pass enough arguments. " ++ docs
+    ["-e"] ->
+      encryptStdin
+    ["-d"] ->
+      decryptStdin
+    [x] ->
+      putStrLn $ "You passed an unsupported option: " ++ x ++ ". " ++ docs
+    _ ->
+      putStrLn $ "You passed too many arguments. " ++ docs
diff --git a/users/wpcarro/scratch/haskell-programming-from-first-principles/monad-transformers.hs b/users/wpcarro/scratch/haskell-programming-from-first-principles/monad-transformers.hs
new file mode 100644
index 0000000000..3a780fc16c
--- /dev/null
+++ b/users/wpcarro/scratch/haskell-programming-from-first-principles/monad-transformers.hs
@@ -0,0 +1,183 @@
+module MonadTransformersScratch where
+
+import Control.Monad
+import qualified Control.Monad.Trans.Maybe as M
+import qualified Control.Monad.Trans.Reader as R
+import qualified Control.Monad.Trans.State as S
+import Data.Function ((&))
+--------------------------------------------------------------------------------
+
+newtype MaybeT m a =
+  MaybeT { runMaybeT :: m (Maybe a) }
+
+instance (Functor f) => Functor (MaybeT f) where
+  fmap f (MaybeT run) =
+    MaybeT $ (fmap . fmap) f run
+
+instance (Applicative m) => Applicative (MaybeT m) where
+  pure x = x & pure & pure & MaybeT
+  _ <*> _ = undefined
+
+instance (Monad m) => Monad (MaybeT m) where
+  return = pure
+  (MaybeT ma) >>= f = MaybeT $ do
+    maybeX <- ma
+    case maybeX of
+      Nothing -> pure Nothing
+      Just x -> x & f & runMaybeT
+
+--------------------------------------------------------------------------------
+
+newtype EitherT e m a =
+  EitherT { runEitherT :: m (Either e a) }
+
+instance (Functor m) => Functor (EitherT e m) where
+  fmap f (EitherT mEither) =
+    EitherT $ (fmap . fmap) f mEither
+
+instance (Applicative m) => Applicative (EitherT e m) where
+  pure x = EitherT $ (pure . pure) x
+  EitherT mEitherF <*> EitherT mEitherX =
+    EitherT $ (fmap (<*>) mEitherF) <*> mEitherX
+
+instance (Monad m) => Monad (EitherT e m) where
+  return = pure
+  EitherT mEitherX >>= f = EitherT $ do
+    eitherX <- mEitherX
+    case eitherX of
+      Left x -> pure $ Left x
+      Right x -> runEitherT $ f x
+
+swapEither :: Either l r -> Either r l
+swapEither (Left x) = Right x
+swapEither (Right x) = Left x
+
+swapEitherT :: (Functor m) => EitherT e m a -> EitherT a m e
+swapEitherT (EitherT mEitherX) =
+  EitherT $ fmap swapEither mEitherX
+
+eitherT :: Monad m => (a -> m c) -> (b -> m c) -> EitherT a m b -> m c
+eitherT aToMC bToMC (EitherT mEitherX) = do
+  eitherX <- mEitherX
+  case eitherX of
+    Left x -> aToMC x
+    Right x -> bToMC x
+
+--------------------------------------------------------------------------------
+
+newtype ReaderT r m a = ReaderT { runReaderT :: r -> m a }
+
+instance (Functor m) => Functor (ReaderT r m) where
+  fmap f (ReaderT rma) =
+    ReaderT $ (fmap . fmap) f rma
+
+instance (Applicative m) => Applicative (ReaderT r m) where
+  pure x = x & pure & pure & ReaderT
+  ReaderT f <*> ReaderT x = ReaderT $ fmap (<*>) f <*> x
+
+-- instance (Monad m) => Monad (ReaderT r m) where
+--   return = pure
+--   ReaderT rma >>= f =
+--     ReaderT $ \r -> do
+--       a <- rma r
+--       runReaderT (f a) r
+-- --------------------------------------------------------------------------------
+
+rDec :: Num a => R.Reader a a
+rDec = R.ReaderT $ \x -> pure $ x + 1
+
+rShow :: Show a => R.Reader a String
+rShow = R.ReaderT $ \x -> pure $ show x
+
+rPrintAndInc :: (Num a, Show a) => R.ReaderT a IO a
+rPrintAndInc = R.ReaderT $ \x ->
+  putStrLn ("Hi: " ++ show x) >> pure (x + 1)
+
+sPrintIncAccum :: (Num a, Show a) => S.StateT a IO String
+sPrintIncAccum = S.StateT $ \x -> do
+  putStrLn ("Hi: " ++ show x)
+  pure (show x, x + 1)
+
+--------------------------------------------------------------------------------
+
+isValid :: String -> Bool
+isValid v = '!' `elem` v
+
+maybeExcite :: M.MaybeT IO String
+maybeExcite = M.MaybeT $ do
+  x <- getLine
+  putStrLn ""
+  case isValid x of
+    False -> pure Nothing
+    True -> pure $ Just x
+
+doExcite :: IO ()
+doExcite = do
+  putStr "Say something *exciting*: "
+  excite <- M.runMaybeT maybeExcite
+  case excite of
+    Nothing -> putStrLn "Gonna need some more excitement..."
+    Just x  -> putStrLn "Now THAT'S exciting...nice!"
+
+--------------------------------------------------------------------------------
+
+data Participant
+  = Man
+  | Machine
+  deriving (Show, Eq)
+
+newtype Hand = Hand (Integer, Integer) deriving (Show, Eq)
+
+newtype Score = Score (Integer, Integer) deriving (Show, Eq)
+
+getLineLn :: String -> IO String
+getLineLn prompt = do
+  putStr prompt
+  x <- getLine
+  putStrLn ""
+  pure x
+
+promptGuess :: IO Hand
+promptGuess = do
+  fingers <- getLineLn "How many fingers (0-5): "
+  guess <- getLineLn "Guess: "
+  pure $ Hand (read guess, read fingers)
+
+aiGuess :: IO Hand
+aiGuess = pure $ Hand (2, 3)
+
+whoWon :: Hand -> Hand -> Maybe Participant
+whoWon (Hand (guessA, fingersA)) (Hand (guessB, fingersB))
+  | guessA == guessB && guessA == (fingersA + fingersB) = Nothing
+  | guessA == (fingersA + fingersB) = Just Man
+  | guessB == (fingersA + fingersB) = Just Machine
+  | otherwise = Nothing
+
+initScore :: Score
+initScore = Score (0, 0)
+
+printScore :: Score -> IO ()
+printScore (Score (man, machine)) =
+  putStrLn $ "Man: " ++ show man ++ " Machine: " ++ show machine
+
+startMorra :: S.StateT Score IO ()
+startMorra = S.StateT $ \(Score (man, machine)) -> do
+  Hand (guessA, fingersA) <- promptGuess
+  Hand (guessB, fingersB) <- aiGuess
+  putStrLn $ "P: " ++ show fingersA ++ "," ++ show guessA
+  putStrLn $ "C: " ++ show fingersB ++ "," ++ show guessB
+  case whoWon (Hand (guessA, fingersA)) (Hand (guessB, fingersB)) of
+    Nothing -> do
+      putStrLn "Nobody won..."
+      printScore (Score (man, machine))
+      pure ((), Score (man, machine))
+    Just Man -> do
+      putStrLn "Man won!"
+      printScore (Score (man + 1, machine))
+      pure ((), Score (man + 1, machine))
+    Just Machine -> do
+      putStrLn "Oh no... Machine won..."
+      printScore (Score (man, machine + 1))
+      pure ((), Score (man, machine + 1))
+
+playMorra = S.runStateT (forever startMorra) initScore
diff --git a/users/wpcarro/scratch/haskell-programming-from-first-principles/monad.hs b/users/wpcarro/scratch/haskell-programming-from-first-principles/monad.hs
new file mode 100644
index 0000000000..2f80b457b1
--- /dev/null
+++ b/users/wpcarro/scratch/haskell-programming-from-first-principles/monad.hs
@@ -0,0 +1,178 @@
+module MonadScratch where
+
+import Data.Function ((&))
+import Test.QuickCheck
+import Test.QuickCheck.Checkers
+import Control.Applicative (liftA2)
+import qualified Control.Monad as Monad
+
+--------------------------------------------------------------------------------
+
+bind :: Monad m => (a -> m b) -> m a -> m b
+bind f x = Monad.join $ fmap f x
+
+--------------------------------------------------------------------------------
+
+fTrigger :: Functor f => f (Int, String, [Int])
+fTrigger = undefined
+
+aTrigger :: Applicative a => a (Int, String, [Int])
+aTrigger = undefined
+
+mTrigger :: Monad m => m (Int, String, [Int])
+mTrigger = undefined
+
+--------------------------------------------------------------------------------
+
+data Sum a b
+  = Fst a
+  | Snd b
+  deriving (Eq, Show)
+
+instance (Eq a, Eq b) => EqProp (Sum a b) where
+  (=-=) = eq
+
+instance (Arbitrary a, Arbitrary b) => Arbitrary (Sum a b) where
+  arbitrary = frequency [ (1, Fst <$> arbitrary)
+                        , (1, Snd <$> arbitrary)
+                        ]
+
+instance Functor (Sum a) where
+  fmap f (Fst x) = Fst x
+  fmap f (Snd x) = Snd (f x)
+
+instance Applicative (Sum a) where
+  pure x = Snd x
+  (Snd f) <*> (Snd x) = Snd (f x)
+  (Snd f) <*> (Fst x) = Fst x
+  (Fst x) <*> _ = Fst x
+
+instance Monad (Sum a) where
+  (Fst x) >>= _ = Fst x
+  (Snd x) >>= f = f x
+
+--------------------------------------------------------------------------------
+
+data Nope a = NopeDotJpg deriving (Eq, Show)
+
+instance Arbitrary (Nope a) where
+  arbitrary = pure NopeDotJpg
+
+instance EqProp (Nope a) where
+  (=-=) = eq
+
+instance Functor Nope where
+  fmap f _ = NopeDotJpg
+
+instance Applicative Nope where
+  pure _ = NopeDotJpg
+  _ <*> _ = NopeDotJpg
+
+instance Monad Nope where
+  NopeDotJpg >>= f = NopeDotJpg
+
+--------------------------------------------------------------------------------
+
+data BahEither b a
+  = PLeft a
+  | PRight b
+  deriving (Eq, Show)
+
+instance (Arbitrary b, Arbitrary a) => Arbitrary (BahEither b a) where
+  arbitrary = frequency [ (1, PLeft <$> arbitrary)
+                        , (1, PRight <$> arbitrary)
+                        ]
+
+instance (Eq a, Eq b) => EqProp (BahEither a b) where
+  (=-=) = eq
+
+instance Functor (BahEither b) where
+  fmap f (PLeft x) = PLeft (f x)
+  fmap _ (PRight x) = PRight x
+
+instance Applicative (BahEither b) where
+  pure = PLeft
+  (PRight x) <*> _ = PRight x
+  (PLeft f) <*> (PLeft x) = PLeft (f x)
+  _ <*> (PRight x) = PRight x
+
+instance Monad (BahEither b) where
+  (PRight x) >>= _ = PRight x
+  (PLeft x) >>= f = f x
+
+--------------------------------------------------------------------------------
+
+newtype Identity a = Identity a
+  deriving (Eq, Ord, Show)
+
+instance Functor Identity where
+  fmap f (Identity x) = Identity (f x)
+
+instance Applicative Identity where
+  pure = Identity
+  (Identity f) <*> (Identity x) = Identity (f x)
+
+instance Monad Identity where
+  (Identity x) >>= f = f x
+
+--------------------------------------------------------------------------------
+
+data List a
+  = Nil
+  | Cons a (List a)
+  deriving (Eq, Show)
+
+instance Arbitrary a => Arbitrary (List a) where
+  arbitrary = frequency [ (1, pure Nil)
+                        , (1, Cons <$> arbitrary <*> arbitrary)
+                        ]
+
+instance Eq a => EqProp (List a) where
+  (=-=) = eq
+
+fromList :: [a] -> List a
+fromList [] = Nil
+fromList (x:xs) = Cons x (fromList xs)
+
+instance Semigroup (List a) where
+  Nil <> xs = xs
+  xs <> Nil = xs
+  (Cons x xs) <> ys =
+    Cons x (xs <> ys)
+
+instance Functor List where
+  fmap f Nil = Nil
+  fmap f (Cons x xs) = Cons (f x) (fmap f xs)
+
+instance Applicative List where
+  pure x = Cons x Nil
+  Nil <*> _ = Nil
+  _ <*> Nil = Nil
+  (Cons f fs) <*> xs =
+    (f <$> xs) <> (fs <*> xs)
+
+instance Monad List where
+  Nil >>= _ = Nil
+  (Cons x xs) >>= f = (f x) <> (xs >>= f)
+
+--------------------------------------------------------------------------------
+
+j :: Monad m => m (m a) -> m a
+j = Monad.join
+
+l1 :: Monad m => (a -> b) -> m a -> m b
+l1 = Monad.liftM
+
+l2 :: Monad m => (a -> b -> c) -> m a -> m b -> m c
+l2 = Monad.liftM2
+
+a :: Monad m => m a -> m (a -> b) -> m b
+a = flip (<*>)
+
+meh :: Monad m => [a] -> (a -> m b) -> m [b]
+meh xs f = flipType $ f <$> xs
+
+flipType :: Monad m => [m a] -> m [a]
+flipType [] = pure mempty
+flipType (m:ms) =
+  m >>= (\x -> (x:) <$> flipType ms)
diff --git a/users/wpcarro/scratch/haskell-programming-from-first-principles/non-strictness.hs b/users/wpcarro/scratch/haskell-programming-from-first-principles/non-strictness.hs
new file mode 100644
index 0000000000..42608fb0c9
--- /dev/null
+++ b/users/wpcarro/scratch/haskell-programming-from-first-principles/non-strictness.hs
@@ -0,0 +1,6 @@
+module NonStrictnessScratch where
+
+x = undefined
+y = "blah"
+main = do
+  print $ snd (x, x `seq` y)
diff --git a/users/wpcarro/scratch/haskell-programming-from-first-principles/reader.hs b/users/wpcarro/scratch/haskell-programming-from-first-principles/reader.hs
new file mode 100644
index 0000000000..7cb7b4a1bb
--- /dev/null
+++ b/users/wpcarro/scratch/haskell-programming-from-first-principles/reader.hs
@@ -0,0 +1,149 @@
+module Reader where
+
+import Data.Char
+import Data.Function ((&))
+import Data.Functor ((<&>))
+import qualified Control.Applicative as A
+import qualified Data.Maybe as MB
+
+cap :: String -> String
+cap xs = xs <&> toUpper
+
+rev :: String -> String
+rev = reverse
+
+compose :: String -> String
+compose xs = xs & rev . cap
+
+fmapped :: String -> String
+fmapped xs = xs & rev <$> cap
+
+tupled :: String -> (String, String)
+tupled xs = A.liftA2 (,) cap rev $ xs
+
+tupled' :: String -> (String, String)
+tupled' = do
+  capResult <- cap
+  revResult <- rev
+  pure (revResult, capResult)
+
+--------------------------------------------------------------------------------
+
+newtype Reader r a = Reader { runReader :: r -> a }
+
+ask :: Reader a a
+ask = Reader id
+
+--------------------------------------------------------------------------------
+
+newtype HumanName = HumanName String
+  deriving (Eq, Show)
+
+newtype DogName = DogName String
+  deriving (Eq, Show)
+
+newtype Address = Address String
+  deriving (Eq, Show)
+
+data Person
+  = Person
+  { humanName :: HumanName
+  , dogName :: DogName
+  , address :: Address
+  } deriving (Eq, Show)
+
+data Dog
+  = Dog
+  { dogsName :: DogName
+  , dogsAddress :: Address
+  } deriving (Eq, Show)
+
+pers :: Person
+pers =
+  Person (HumanName "Big Bird")
+         (DogName "Barkley")
+         (Address "Sesame Street")
+
+chris :: Person
+chris =
+  Person (HumanName "Chris Allen")
+         (DogName "Papu")
+         (Address "Austin")
+
+getDog :: Person -> Dog
+getDog p =
+  Dog (dogName p) (address p)
+
+getDogR :: Person -> Dog
+getDogR =
+  A.liftA2 Dog dogName address
+
+--------------------------------------------------------------------------------
+
+myLiftA2 :: Applicative f => (a -> b -> c) -> f a -> f b -> f c
+myLiftA2 f x y =
+  f <$> x <*> y
+
+asks :: (r -> a) -> Reader r a
+asks f = Reader f
+
+--------------------------------------------------------------------------------
+
+instance Functor (Reader a) where
+  fmap f (Reader ab) = Reader $ f . ab
+
+instance Applicative (Reader a) where
+  pure x = Reader $ \_ -> x
+  (Reader rab) <*> (Reader ra) = Reader $ do
+    ab <- rab
+    fmap ab ra
+
+--------------------------------------------------------------------------------
+
+instance Monad (Reader r) where
+  return = pure
+  -- (>>=) :: Reader r a -> (a -> Reader r b) -> Reader r b
+  (Reader x) >>= f = undefined
+
+--------------------------------------------------------------------------------
+
+x = [1..3]
+y = [4..6]
+z = [7..9]
+
+xs :: Maybe Integer
+xs = zip x y & lookup 3
+
+ys :: Maybe Integer
+ys = zip y z & lookup 6
+
+zs :: Maybe Integer
+zs = zip x y & lookup 4
+
+z' :: Integer -> Maybe Integer
+z' n = zip x y & lookup n
+
+x1 :: Maybe (Integer, Integer)
+x1 = A.liftA2 (,) xs ys
+
+x2 :: Maybe (Integer, Integer)
+x2 = A.liftA2 (,) ys zs
+
+x3 :: Integer -> (Maybe Integer, Maybe Integer)
+x3 n = (z' n, z' n)
+
+summed :: Num a => (a, a) -> a
+summed (x, y) = x + y
+
+bolt :: Integer -> Bool
+bolt x = x > 3 && x < 8
+
+main :: IO ()
+main = do
+  print $ sequenceA [Just 3, Just 2, Just 1]
+  print $ sequenceA [x, y]
+  print $ sequenceA [xs, ys]
+  print $ summed <$> ((,) <$> xs <*> ys)
+  print $ bolt 7
+  print $ bolt <$> z
+  print $ sequenceA [(>3), (<8) ,even] 7
diff --git a/users/wpcarro/scratch/haskell-programming-from-first-principles/shell.nix b/users/wpcarro/scratch/haskell-programming-from-first-principles/shell.nix
new file mode 100644
index 0000000000..b594a4207e
--- /dev/null
+++ b/users/wpcarro/scratch/haskell-programming-from-first-principles/shell.nix
@@ -0,0 +1,8 @@
+let
+  briefcase = import <briefcase> {};
+in briefcase.buildHaskell.shell {
+  deps = hpkgs: with hpkgs; [
+    quickcheck-simple
+    checkers
+  ];
+}
diff --git a/users/wpcarro/scratch/haskell-programming-from-first-principles/state.hs b/users/wpcarro/scratch/haskell-programming-from-first-principles/state.hs
new file mode 100644
index 0000000000..f63e0ecdf1
--- /dev/null
+++ b/users/wpcarro/scratch/haskell-programming-from-first-principles/state.hs
@@ -0,0 +1,93 @@
+module StateScratch where
+
+--------------------------------------------------------------------------------
+import System.Random
+-- import Control.Monad.Trans.State
+import Data.Function ((&))
+
+import qualified Control.Applicative as Ap
+import qualified Control.Monad as M
+--------------------------------------------------------------------------------
+
+data Die
+  = DieOne
+  | DieTwo
+  | DieThree
+  | DieFour
+  | DieFive
+  | DieSix
+  deriving (Eq, Show)
+
+intToDie :: Integer -> Maybe Die
+intToDie 1 = Just DieOne
+intToDie 2 = Just DieTwo
+intToDie 3 = Just DieThree
+intToDie 4 = Just DieFour
+intToDie 5 = Just DieFive
+intToDie 6 = Just DieSix
+intToDie _ = Nothing
+
+rollDie :: Moi StdGen Die
+rollDie = do
+  (n, s) <- randomR (1, 6)
+  case intToDie n of
+    Just d  -> pure (d, s)
+    Nothing -> pure (DieOne, s)
+
+rollsToGetN :: Integer -> StdGen -> [Die]
+rollsToGetN n g = go 0 [] g
+  where
+    go sum result gen
+      | sum >= n = result
+      | otherwise =
+        let (dice, nextGen) = randomR (1, 6) gen
+        in case intToDie dice of
+          Nothing -> go (sum + dice) result nextGen
+          Just d  -> go (sum + dice) (d : result) nextGen
+
+--------------------------------------------------------------------------------
+
+newtype Moi s a = Moi { runMoi :: s -> (a, s) }
+
+instance Functor (Moi s) where
+  fmap f (Moi run) =
+    Moi $ \s -> let (x, t) = run s
+                in (f x, t)
+
+instance Applicative (Moi s) where
+  pure x = Moi $ \s -> (x, s)
+  (Moi f) <*> (Moi run) =
+    Moi $ \s -> let (g, t) = f s
+                    (x, u) = run t
+                in (g x, u)
+
+instance Monad (Moi s) where
+  (Moi run1) >>= f =
+    Moi $ \s -> let (x, t) = run1 s
+                    (Moi run2) = f x
+                in run2 t
+
+--------------------------------------------------------------------------------
+
+fizzBuzz :: Integer -> String
+fizzBuzz n | n `mod` 15 == 0 = "FizzBuzz"
+           | n `mod`  5 == 0 = "Buzz"
+           | n `mod`  3 == 0 = "Fizz"
+           | otherwise       = show n
+
+--------------------------------------------------------------------------------
+
+get :: Moi s s
+get = Moi $ \s -> (s, s)
+
+put :: s -> Moi s ()
+put x = Moi $ \s -> ((), x)
+
+exec :: Moi s a -> s -> s
+exec (Moi run) x = x & run & snd
+
+eval :: Moi s a -> s -> a
+eval (Moi run) x = x & run & fst
+
+modify :: (s -> s) -> Moi s ()
+modify f = Moi $ \s -> ((), f s)
diff --git a/users/wpcarro/scratch/haskell-programming-from-first-principles/traversable.hs b/users/wpcarro/scratch/haskell-programming-from-first-principles/traversable.hs
new file mode 100644
index 0000000000..5dc4ea411b
--- /dev/null
+++ b/users/wpcarro/scratch/haskell-programming-from-first-principles/traversable.hs
@@ -0,0 +1,131 @@
+module TraversableScratch where
+
+import qualified Data.Foldable as F
+
+import Test.QuickCheck
+
+newtype Identity a = Identity a
+  deriving (Eq, Ord, Show)
+
+instance Functor Identity where
+  fmap f (Identity x) = Identity (f x)
+
+instance Foldable Identity where
+  foldMap f (Identity x) = f x
+
+instance Traversable Identity where
+  traverse f (Identity x) = Identity <$> f x
+
+--------------------------------------------------------------------------------
+
+data Optional a
+  = Nada
+  | Some a
+  deriving (Eq, Show)
+
+instance Functor Optional where
+  fmap f Nada = Nada
+  fmap f (Some x) = Some (f x)
+
+instance Foldable Optional where
+  foldMap f Nada = mempty
+  foldMap f (Some x) = f x
+
+instance Traversable Optional where
+  traverse f Nada = pure Nada
+  traverse f (Some x) = Some <$> f x
+
+--------------------------------------------------------------------------------
+
+data List a = Nil | Cons a (List a) deriving (Eq, Show)
+
+instance Functor List where
+  fmap _ Nil = Nil
+  fmap f (Cons x xs) = Cons (f x) (fmap f xs)
+
+instance Foldable List where
+  foldMap f Nil = mempty
+  foldMap f (Cons x xs) = mappend (f x) (foldMap f xs)
+
+instance Traversable List where
+  sequenceA Nil = pure Nil
+  sequenceA (Cons x xs) = Cons <$> x <*> sequenceA xs
+
+--------------------------------------------------------------------------------
+
+data Three a b c = Three a b c
+  deriving (Eq, Show)
+
+instance Functor (Three a b) where
+  fmap f (Three x y z) = Three x y (f z)
+
+instance Foldable (Three a b) where
+  foldMap f (Three _ _ z) = f z
+
+instance Traversable (Three a b) where
+  sequenceA (Three x y z) = (\z' -> Three x y z') <$> z
+
+--------------------------------------------------------------------------------
+
+data Pair a b = Pair a b
+  deriving (Eq, Show)
+
+instance Functor (Pair a) where
+  fmap f (Pair x y) = Pair x (f y)
+
+instance Foldable (Pair a) where
+  foldMap f (Pair x y) = f y
+
+instance Traversable (Pair a) where
+  sequenceA (Pair x y) = (\y' -> Pair x y') <$> y
+
+--------------------------------------------------------------------------------
+
+data Big a b = Big a b b
+  deriving (Eq, Show)
+
+instance Functor (Big a) where
+  fmap f (Big x y z) = Big x (f y) (f z)
+
+instance Foldable (Big a) where
+  foldMap f (Big x y z) = f y <> f z
+
+instance Traversable (Big a) where
+  sequenceA (Big x y z) = (\y' z' -> Big x y' z') <$> y <*> z
+
+--------------------------------------------------------------------------------
+
+data Bigger a b = Bigger a b b b
+  deriving (Eq, Show)
+
+instance Functor (Bigger a) where
+  fmap f (Bigger w x y z) = Bigger w (f x) (f y) (f z)
+
+instance Foldable (Bigger a) where
+  foldMap f (Bigger w x y z) = f x <> f y <> f z
+
+instance Traversable (Bigger a) where
+  sequenceA (Bigger w x y z) = (\x' y' z' -> Bigger w x' y' z') <$> x <*> y <*> z
+
+--------------------------------------------------------------------------------
+
+data Tree a
+  = Empty
+  | Leaf a
+  | Node (Tree a) a (Tree a)
+  deriving (Eq, Show)
+
+instance Functor Tree where
+  fmap f Empty = Empty
+  fmap f (Leaf x) = Leaf (f x)
+  fmap f (Node lhs x rhs) = Node (fmap f lhs) (f x) (fmap f rhs)
+
+instance Foldable Tree where
+  foldMap f Empty = mempty
+  foldMap f (Leaf x) = f x
+  foldMap f (Node lhs x rhs) = (foldMap f lhs) <> (f x) <> (foldMap f rhs)
+
+instance Traversable Tree where
+  sequenceA Empty = pure Empty
+  sequenceA (Leaf x) = Leaf <$> x
+  sequenceA (Node lhs x rhs) = Node <$> sequenceA lhs <*> x <*> sequenceA rhs
author	Vincent Ambo <mail@tazj.in>	2021-12-13T22·51+0300
committer	Vincent Ambo <mail@tazj.in>	2021-12-13T23·15+0300
commit	019f8fd2113df4c5247c3969c60fd4f0e08f91f7 (patch)
tree	76a857f61aa88f62a30e854651e8439db77fd0ea /users/wpcarro/scratch
parent	464bbcb15c09813172c79820bcf526bb10cf4208 (diff)
parent	6123e976928ca3d8d93f0b2006b10b5f659eb74d (diff)