389. Find the Difference

class Solution {public:    char findTheDifference(string s, string t) {        map<char,int> tm;        map<char,int> sm;        for(int i=0;i<t.size();i++)            tm[t[i]]++;        for(int i=0;i<s.size();i++)            sm[s[i]]++;        for(map<char,int>::iterator it=tm.begin();it!=tm.end();it++) {            if (sm[it->first] == 0 || sm[it->first] != it->second)                return it->first;        }        return ' ';    }};