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.

思路：先用hashmap统计词频，然后用pq来建立string；

class Solution {
    public class Node {
        public char c;
        public int fre;
        public Node(char c, int fre) {
            this.c = c;
            this.fre = fre;
        }
    }
    
    public String frequencySort(String s) {
        HashMap<Character, Integer> hashmap = new HashMap<>();
        for(int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            hashmap.put(c, hashmap.getOrDefault(c, 0) + 1);
        }
        
        PriorityQueue<Node> pq = new PriorityQueue<Node>((a, b) -> (b.fre - a.fre));
        for(Character c: hashmap.keySet()) {
            pq.offer(new Node(c, hashmap.get(c)));
        }
        
        StringBuilder sb = new StringBuilder();
        while(!pq.isEmpty()) {
            Node node = pq.poll();
            int k = node.fre;
            while(k > 0) {
                sb.append(node.c);
                k--;
            }
        }
        return sb.toString();
    }
}

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