public String frequencySort(String s) {

         HashMap<Character,Integer> map = new HashMap<>();

         for(int i = 0; i < s.length(); i++){

             char c = s.charAt(i);

             map.put(c, map.getOrDefault(c,0) + 1);

         }

         List<Character>[] frequency = new ArrayList[s.length() + 1];

         for(char key : map.keySet()){

             int index = map.get(key);

             if(frequency[index] == null){

                 frequency[index] = new ArrayList<>();

             }

             frequency[index].add(key);

         }

         StringBuffer ans = new StringBuffer();

         for(int i = frequency.length - 1; i >= 0; i--){

             if(frequency[i] == null) continue;

             for(char c : frequency[i]){

                 for(int count = 0; count < i; count++){

                     ans.append(c);

                 }

             }

         }

         return ans.toString();

     }