Add str diff challenge

arrays_strings/str_diff/test_str_diff.py

@@ -0,0 +1,20 @@
+from nose.tools import assert_equal, assert_raises
+
+
+class TestFindDiff(object):
+
+    def test_find_diff(self):
+        solution = Solution()
+        assert_raises(TypeError, solution.find_diff, None)
+        assert_equal(solution.find_diff('abcd', 'abcde'), 'e')
+        assert_equal(solution.find_diff('aaabbcdd', 'abdbacade'), 'e')
+        print('Success: test_find_diff')
+
+
+def main():
+    test = TestFindDiff()
+    test.test_find_diff()
+
+
+if __name__ == '__main__':
+    main()