      "\n",
      "* None input -> TypeError\n",
      "* 'abcd', 'abcde' -> 'e'\n",
      "* 'aaabbcdd', 'abdbacade' -> 'e'"
     ]
    },
    {
 
</s> remove     "    def find_diff(self, s, t):\n",
 </s> add     "    def find_diff(self, str1, str2):\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 
    "* 'ab', 'aab' -> 'a'\n",
    "* 'aab', 'ab' -> 'a'\n",
      "\n",
      "* None input -> TypeError\n",
    "* 'ab', 'aab' -> 'a'\n",
    "* 'aab', 'ab' -> 'a'\n",
      "* 'abcd', 'abcde' -> 'e'\n",
      "* 'aaabbcdd', 'abdbacade' -> 'e'"
     ]
    },
    {
keep add 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