C++map值排序

class Solution {
public:
    static bool cmp(pair a, pair b){
        return a.second>b.second;
    }
    string frequencySort(string s) {
        string ans="";
        map mp;
        for(auto c:s){
            mp[c]++;
        }
        vector> vs;
        for(auto kv:mp){
            vs.push_back({kv.first,kv.second});
        }
        sort(vs.begin(),vs.end(),cmp);
        for(auto kv:vs){
            for(int i=0;i