Leetcode.451 Sort Characters By Frequency（Java）

Leetcode.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.

Solution

优先队列

class Solution {
    public String frequencySort(String s) {
        Map<Character,Integer> map = new HashMap<>();
        for(char c:s.toCharArray()){
            map.put(c, map.getOrDefault(c,0)+1);
        }
        PriorityQueue<Character> pq = new PriorityQueue<>(new Comparator<Character>(){
            @Override
            public int compare(Character c1,Character c2){
                return map.get(c2) - map.get(c1);
            }
        });
        for(Character c: map.keySet()){
            pq.add(c);
        }
        StringBuilder res = new StringBuilder();
        while(!pq.isEmpty()){
            Character c = pq.poll();
            int n = map.get(c);
            while(n>0){
                res.append(c);
                n--;
            }
        }
        return res.toString();
    }
}