     "metadata": {},
     "source": [
      "## Constraints\n",
      "\n",
      "* Assume two strings str1, str2, where str2 contains the same set of characters in str1 with one additional character.\n",
      "* Can we assume the strings are ASCII?\n",
      "    * Yes\n",
      "* Is case important?\n",
      "    * The strings are lower case\n",
      "* Can we assume the inputs are valid?\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",
     "metadata": {},
     "source": [
      "## Constraints\n",
      "\n",

      "* Can we assume the strings are ASCII?\n",
      "    * Yes\n",
      "* Is case important?\n",
      "    * The strings are lower case\n",
      "* Can we assume the inputs are valid?\n",
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