Given a string, sort it in decreasing order based on the frequency of characters.

Example 1:

Input:"tree"Output:"eert"Explanation:'e' appears twice while 'r' and 't' both appear once.So 'e' must appear before both 'r' and 't'. Therefore "eetr" is also a valid answer.

 

Example 2:

Input:"cccaaa"Output:"cccaaa"Explanation:Both 'c' and 'a' appear three times, so "aaaccc" is also a valid answer.Note that "cacaca" is incorrect, as the same characters must be together.

Example 3:

Input:"Aabb"Output:"bbAa"Explanation:"bbaA" is also a valid answer, but "Aabb" is incorrect.Note that 'A' and 'a' are treated as two different characters. 题目含义：按字母出现次数的降序重排字符串

1     public String frequencySort(String s) { 2         Map
     
       map = new HashMap<>(); 3         for (int i = 0; i < s.length(); i++) { 4             map.put(s.charAt(i), map.getOrDefault(s.charAt(i), 0)+1); 5         } 6         PriorityQueue
      
       
        > pq = new PriorityQueue<>( 7                 new Comparator
        
         
          >() { 8                     @Override 9                     public int compare(Map.Entry
          
            a, Map.Entry
           
             b) {10 return b.getValue() - a.getValue();11 }12 }13 );14 pq.addAll(map.entrySet());15 StringBuilder sb = new StringBuilder();16 while (!pq.isEmpty()) {17 Map.Entry
            
              e = pq.poll();18 for (int i = 0; i < e.getValue(); i++) {19 sb.append(e.getKey());20 }21 }22 return sb.toString(); 23 }