          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')
          assert_equal(solution.find_diff_xor('ab', 'aab'), 'a')
          assert_equal(solution.find_diff_xor('aab', 'ab'), 'a')
 
</s> remove     "        assert_raises(TypeError, solution.find_diff, None, None)\n",
 </s> add     "        assert_raises(TypeError, solution.find_diff, None)\n",
    "        assert_equal(solution.find_diff('ab', 'aab'), 'a')\n",
    "        assert_equal(solution.find_diff('aab', 'ab'), 'a')\n",
        assert_equal(solution.find_diff('ab', 'aab'), 'a')
        assert_equal(solution.find_diff('aab', 'ab'), 'a')
          solution = Solution()
          assert_raises(TypeError, solution.find_diff, None)
        assert_equal(solution.find_diff('ab', 'aab'), 'a')
        assert_equal(solution.find_diff('aab', 'ab'), 'a')
          assert_equal(solution.find_diff('abcd', 'abcde'), 'e')
          assert_equal(solution.find_diff('aaabbcdd', 'abdbacade'), 'e')
          assert_equal(solution.find_diff_xor('ab', 'aab'), 'a')
          assert_equal(solution.find_diff_xor('aab', 'ab'), 'a')
keep add keep keep keep keep
 Update str diff challenge (#242)

Update constraints, test cases, and dictionary solution. Fix inconsistencies between challenge and solution notebooks. 
arrays_strings/str_diff/test_str_diff.py