HEAP_451

class Solution {
    public String frequencySort(String s) {
        HashMap<Character, Integer> map = new HashMap<>();
        for(char ch : s.toCharArray()){
            map.put(ch, map.getOrDefault(ch, 0) + 1);
        }
        PriorityQueue<Character> pq = new PriorityQueue<>(
            (a, b) -> map.get(b) - map.get(a)
        );
        for(char ch : map.keySet()){
            pq.offer(ch);
        }
        StringBuilder ans = new StringBuilder();
        while(!pq.isEmpty()){
            char ch = pq.poll();
            int freq = map.get(ch);
            while(freq-- > 0){
                ans.append(ch);
            }
        }
        return ans.toString();
    }
}