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)
