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.

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();
      } 
}

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