Solve InterviewCake's "find rotation point" problem

Write a function that accepts a rotated cycle of alphabetically sorted strings
and returns the index what should be the first element if the elements were not
rotated.

2 files changed, 69 insertions, 1 deletions

diff --git a/scratch/deepmind/part_two/find-rotation-point.ts b/scratch/deepmind/part_two/find-rotation-point.ts
new file mode 100644
index 000000000000..7bf1a484454d
--- /dev/null
+++ b/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/scratch/deepmind/part_two/todo.org b/scratch/deepmind/part_two/todo.org
index 228dd08ef3f6..aeb890de5579 100644
--- a/scratch/deepmind/part_two/todo.org
+++ b/scratch/deepmind/part_two/todo.org
@@ -16,7 +16,7 @@
 ** DONE Cafe Order Checker
 ** DONE In-Place Shuffle
 * Sorting, searching, and logarithms
-** TODO Find Rotation Point
+** DONE Find Rotation Point
 ** TODO Find Repeat, Space Edition
 ** TODO Top Scores
 ** TODO Merging Meeting Times