451. Sort Characters By Frequency

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.

hashmap 按照出现的频率排序。

public class Solution {class Cha{public char  c;public int i;public Cha(char c,int i){this.c=c;this.i=i;}}class compare implements Comparator<Cha>{public int compare(Cha c1,Cha c2){return c2.i-c1.i;}}    public String frequencySort(String s) {        if(s.length()==0) return new String();                Map<Character,Integer> cha=new HashMap<>();        for(int i=0;i<s.length();i++){        Integer freq=cha.get(s.charAt(i));        cha.put(s.charAt(i), freq==null?1:freq+1);        }                PriorityQueue<Cha> pq=new PriorityQueue<>(new compare());        for(Character e:cha.keySet()){        pq.add(new Cha(e,cha.get(e)));        }        StringBuilder  sb=new StringBuilder();        while(!pq.isEmpty()){        Cha p=pq.poll();        for(int i=0;i<p.i;i++){        sb.append(p.c);        }        }        return sb.toString();      } }