      "* Loop through t, decrementing the seen values\n",
      "    * If the char is not there or if the decrement results in a negative value, return the char\n",
      "\n",
      "Complexity:\n",
      "* Time: O(m+n), where m and n are the lengths of s, t\n",
      "* Space: O(h), for the dict, where h is the unique chars in s\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", </s> remove     "    def find_diff(self, s, t):\n",
 </s> add     "    def find_diff(self, str1, str2):\n",
    "* Return the differing char from the dictionary\n",
      "* Loop through t, decrementing the seen values\n",
      "    * If the char is not there or if the decrement results in a negative value, return the char\n",
    "* Return the differing char from the dictionary\n",
      "\n",
      "Complexity:\n",
      "* Time: O(m+n), where m and n are the lengths of s, t\n",
      "* Space: O(h), for the dict, where h is the unique chars in s\n",
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/str_diff_solution.ipynb