      "    def test_find_diff(self):\n",
      "        solution = Solution()\n",
      "        assert_raises(TypeError, solution.find_diff, None)\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",
 
</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",
    "        assert_equal(solution.find_diff('ab', 'aab'), 'a')\n",
    "        assert_equal(solution.find_diff('aab', 'ab'), 'a')\n",
      "    def test_find_diff(self):\n",
      "        solution = Solution()\n",
      "        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('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",
keep keep add keep 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_solution.ipynb