1 files changed, 33 insertions, 0 deletions
diff --git a/universe/data_structures_and_algorithms/which-appears-twice.py b/universe/data_structures_and_algorithms/which-appears-twice.py
new file mode 100644
index 000000000000..e9a4f0eb24d0
--- /dev/null
+++ b/universe/data_structures_and_algorithms/which-appears-twice.py
@@ -0,0 +1,33 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+# find_repeat :: [Int] -> Int
+def find_repeat(xs):
+    n = len(xs) - 1
+    return sum(xs) - ((n**2 + n) / 2)
+
+
+################################################################################
+# 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)