451. 根据字符出现频率排序（排序）（堆排序）

方法一： 哈希表+堆排序

class Solution {
    public String frequencySort(String s) {
        HashMap<Character,Integer> map = new HashMap<>();

        for(int i =0;i<s.length();i++){
            map.put(s.charAt(i),map.getOrDefault(s.charAt(i),0)+1);
        }

        PriorityQueue<Character> heap = new PriorityQueue<>((n1,n2)->(map.get(n2)-map.get(n1)));
        for(Character ch : map.keySet()){
            heap.add(ch);
        }
        StringBuilder str = new StringBuilder();
        while(!heap.isEmpty()){
            Character tmpCh = heap.poll();
            int sum = map.get(tmpCh);
            for(int i = 0;i<sum;i++)
                str.append(tmpCh);
        }
        
        return str.toString();
    }
}