     "outputs": [],
     "source": [
      "class Solution(object):\n",
      "\n",
      "    def find_diff(self, s, t):\n",
      "        # TODO: Implement me\n",
      "        pass"
     ]
    },
    {
 
</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     "* Assume two strings str1, str2, where str2 contains the same set of characters in str1 with one additional character.\n",
 </s> add  </s> remove     "        return None\n",
 </s> add     "        for char, count in seen.items():\n",
    "            return char\n",
    "    def find_diff(self, str1, str2):\n",
     "outputs": [],
     "source": [
      "class Solution(object):\n",
      "\n",
    "    def find_diff(self, str1, str2):\n",
      "        # TODO: Implement me\n",
      "        pass"
     ]
    },
    {
keep keep keep keep replace 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