LeetCode 451. Sort Characters By Frequency(按照字符出现次数对字符串排序)

题意：按照字符出现次数对字符串排序。

class Solution {
public:
    map<char, int> mp;
    vector<char> v[1000010]; 
    string frequencySort(string s) {
        int len = s.size();
        for(int i = 0; i < len; ++i){
            ++mp[s[i]];
        }
        int ma = 0;
        for(map<char, int>::iterator it = mp.begin(); it != mp.end(); ++it){
            v[it -> second].push_back(it -> first);
            ma = max(ma, it -> second);
        }
        string ans = "";
        for(int i = ma; i >= 1; --i){
            len = v[i].size();
            for(int j = 0; j < len; ++j){
                for(int k = 0; k < i; ++k){
                    ans += v[i][j];
                }
            }
        }
        return ans;
    }
};