Complete which-appears-twice

Solves the InterviewCake.com problem that asks us to write a function that
returns the number, y, that occurs twice in a list, xs, where xs is an unsorted
list of integers from 1..n with a length of n + 1.

1 files changed, 29 insertions, 0 deletions

diff --git a/deepmind/which-appears-twice.py b/deepmind/which-appears-twice.py
new file mode 100644
index 000000000000..c01379295d32
--- /dev/null
+++ b/deepmind/which-appears-twice.py
@@ -0,0 +1,29 @@
+import unittest
+
+
+def find_repeat(xs):
+    n = max(xs)
+    expected_sum = (n + 1) * n / 2
+    actual_sum = sum(xs)
+    return actual_sum - expected_sum
+
+
+# Tests
+class Test(unittest.TestCase):
+    def test_short_list(self):
+        actual = find_repeat([1, 2, 1])
+        expected = 1
+        self.assertEqual(actual, expected)
+
+    def test_medium_list(self):
+        actual = find_repeat([4, 1, 3, 4, 2])
+        expected = 4
+        self.assertEqual(actual, expected)
+
+    def test_long_list(self):
+        actual = find_repeat([1, 5, 9, 7, 2, 6, 3, 8, 2, 4])
+        expected = 2
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)