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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)
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