static bool cmp(pair<char, int> a , pair<char,int> b)

       {

            return a.second>b.second; //按照value从大到小重新排序

       }

    string frequencySort(string s) {

        unordered_map<char,int> m;

        for(auto i:s)

        m[i]++;

        vector<pair<char,int>> v;

        for(auto i=m.begin();i!=m.end();i++)

        {

            v.push_back(pair<char,int>(i->first, i->second));

        }

        sort(v.begin(),v.end(),cmp);

        return s;

}