https://leetcode.com/problems/find-the-difference/description/
class Solution {
public:
char findTheDifference(string s, string t) {
map<char, int> Mp;
for(int i=0; i<s.size(); i++) Mp[s[i]] += 1;
for(int i=0; i<t.size(); i++){
if(Mp[t[i]] !=0) Mp[t[i]] -= 1;
else return t[i];
}
return 'a';
}
};