      "        assert_equal(solution.find_diff('aab', 'ab'), 'a')\n",
      "        assert_equal(solution.find_diff('abcd', 'abcde'), 'e')\n",
      "        assert_equal(solution.find_diff('aaabbcdd', 'abdbacade'), 'e')\n",
      "        print('Success: test_find_diff')\n",
      "\n",
      "\n",
      "def main():\n",
      "    test = TestFindDiff()\n",
 
</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", </s> remove     "        return None\n",
 </s> add     "        for char, count in seen.items():\n",
    "            return char\n", </s> remove     "    def find_diff(self, s, t):\n",
 </s> add     "    def find_diff(self, str1, str2):\n",
    "        assert_equal(solution.find_diff_xor('ab', 'aab'), 'a')\n",
    "        assert_equal(solution.find_diff_xor('aab', 'ab'), 'a')\n",
    "        assert_equal(solution.find_diff_xor('abcd', 'abcde'), 'e')\n",
    "        assert_equal(solution.find_diff_xor('aaabbcdd', 'abdbacade'), 'e')\n",
      "        assert_equal(solution.find_diff('aab', 'ab'), 'a')\n",
      "        assert_equal(solution.find_diff('abcd', 'abcde'), 'e')\n",
      "        assert_equal(solution.find_diff('aaabbcdd', 'abdbacade'), 'e')\n",
    "        assert_equal(solution.find_diff_xor('ab', 'aab'), 'a')\n",
    "        assert_equal(solution.find_diff_xor('aab', 'ab'), 'a')\n",
    "        assert_equal(solution.find_diff_xor('abcd', 'abcde'), 'e')\n",
    "        assert_equal(solution.find_diff_xor('aaabbcdd', 'abdbacade'), 'e')\n",
      "        print('Success: test_find_diff')\n",
      "\n",
      "\n",
      "def main():\n",
      "    test = TestFindDiff()\n",
keep keep add keep 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/str_diff_challenge.ipynb