class Solution {

 public:

     string frequencySort(string s) {

         map<char, int> mp;

         for (auto c : s) {

             mp[c]++;

         }

         string ans = "";

         // 桶排序

         vector<string> bucket(s.size()+, "");

         for (auto ele : mp) {

             char ch = ele.first;

             int cnt = ele.second;

             bucket[cnt].append(cnt, ch);

         }

         for (auto i = bucket.size() - ; i >= ; --i) {

             ans += bucket[i];

         }

         return ans;

     }

 };