Add InterviewCake.com examples

Adds some of the code I generated while studying for a role transfer at Google
using the fantastic resource, InterviewCake.com. This work predates the
mono-repo.

I should think of ways to DRY up this code and the code in
crack_the_coding_interview, but I'm afraid I'm creating unnecessary work for
myself that way.

data_structures_and_algorithms/find-unique-int-among-duplicates.py

@@ -0,0 +1,45 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+def find_unique_delivery_id(xs):
+    a = 0
+    for x in xs:
+        a ^= x
+    return a
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_one_drone(self):
+        actual = find_unique_delivery_id([1])
+        expected = 1
+        self.assertEqual(actual, expected)
+
+    def test_unique_id_comes_first(self):
+        actual = find_unique_delivery_id([1, 2, 2])
+        expected = 1
+        self.assertEqual(actual, expected)
+
+    def test_unique_id_comes_last(self):
+        actual = find_unique_delivery_id([3, 3, 2, 2, 1])
+        expected = 1
+        self.assertEqual(actual, expected)
+
+    def test_unique_id_in_middle(self):
+        actual = find_unique_delivery_id([3, 2, 1, 2, 3])
+        expected = 1
+        self.assertEqual(actual, expected)
+
+    def test_many_drones(self):
+        actual = find_unique_delivery_id(
+            [2, 5, 4, 8, 6, 3, 1, 4, 2, 3, 6, 5, 1])
+        expected = 8
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)