Splice ./universe directory into ./

Manually merging:
- README.md: I added the description from universe/README.md into the heading of
  dotfiles/README.md.
- .envrc: dotfiles/.envrc was a superset of universe/.envrc
- .gitignore: Adding some of the ignored patterns from universe/.gitignore to
  dotfiles/.gitignore

Everything else here should be a simple rename.

1 files changed, 88 insertions, 0 deletions

diff --git a/data_structures_and_algorithms/linked-list-cycles.py b/data_structures_and_algorithms/linked-list-cycles.py
new file mode 100644
index 000000000000..75a4b993944c
--- /dev/null
+++ b/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)