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

git-subtree-dir: users/wpcarro
git-subtree-mainline: 464bbcb15c
git-subtree-split: 24f5a642af
Change-Id: I6105b3762b79126b3488359c95978cadb3efa789

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)