leetcode.排序.451根据字符出现频率排序-Java

 1 public String frequencySort(String s) {
 2         HashMap<Character,Integer> map = new HashMap<>();
 3         for(int i = 0; i < s.length(); i++){
 4             char c = s.charAt(i);
 5             map.put(c, map.getOrDefault(c,0) + 1);
 6         }
 7         List<Character>[] frequency = new ArrayList[s.length() + 1];
 8         for(char key : map.keySet()){
 9             int index = map.get(key);
10             if(frequency[index] == null){
11                 frequency[index] = new ArrayList<>();
12             }
13             frequency[index].add(key); 
14         }
15         StringBuffer ans = new StringBuffer();
16         for(int i = frequency.length - 1; i >= 0; i--){
17             if(frequency[i] == null) continue;
18             for(char c : frequency[i]){
19                 for(int count = 0; count < i; count++){
20                     ans.append(c);
21                 }
22             }
23         }
24         return ans.toString();
25     }