public class Solution {

     public String frequencySort(String s) {

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

         char[] letters = s.toCharArray();

         for(char l:letters) {

             if(map.containsKey(l)) {map.put(l, map.get(l)+1);}

             else {map.put(l, 1);}

         }

         PriorityQueue<Map.Entry<Character,Integer>> pq = new PriorityQueue<>(

                 new Comparator<Map.Entry<Character,Integer>>(){

                     @Override

                     public int compare(Map.Entry<Character,Integer> a,Map.Entry<Character,Integer> b) {

                         return b.getValue()-a.getValue();

                     }

                 }

         );

         pq.addAll(map.entrySet());

         StringBuilder sb = new StringBuilder();

         while(!pq.isEmpty()) {

             Map.Entry<Character, Integer> e = pq.poll();

             for(int i=0;i<e.getValue();i++) sb.append(e.getKey());

         }

         return sb.toString();

     }

 }