Partition deepmind directory into two parts

Since I did not pass my one-site interview with DM, but I have been invited to
attempt again, I decided to partition this directory into two parts:
1. part_one: Hosting the exercises that I completed before my first attempt at
   earning the job.
2. part_two: Hosting the exercise that I will complete before my second attempt
   at earning the job.

deepmind/part_one/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)