      "    * The strings are lower case\n",
      "* Can we assume the inputs are valid?\n",
      "    * No, check for None\n",
      "* Can we assume this fits memory?\n",
      "    * Yes"
     ]
    },
 
</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",
    "    * Otherwise, assume there is only a single different char between the two strings\n",
      "    * The strings are lower case\n",
      "* Can we assume the inputs are valid?\n",
      "    * No, check for None\n",
    "    * Otherwise, assume there is only a single different char between the two strings\n",
      "* Can we assume this fits memory?\n",
      "    * Yes"
     ]
    },
keep 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