leetcode 451. Sort Characters By Frequency

题目：

Given a string, sort it in decreasing order based on the frequency of characters.

Example 1:

Input:
"tree"

Output:
"eert"

Explanation:
'e' appears twice while 'r' and 't' both appear once.
So 'e' must appear before both 'r' and 't'. Therefore "eetr" is also a valid answer.

class Solution {
public:
    string frequencySort(string s) {
        string t;
        int len = s.size();
        map<char,int> re;
        for(int i = 0; i < len; i++){
            re[s[i]]++;
        }
        while(1){
            int flag = -1;
            char max ;
            for(auto val:re){
                if(val.second>flag) max = val.first,flag = val.second;
            }
            if(flag == -1) return t;
            while(flag--){
                t+=max;
            }
            re[max] = -1;
        }
    }
};