[Level 1] 문자열 내림차순으로 배치하기

import java.util.*;

class Solution {
    public String solution(String s) {
        String answer = "";
        PriorityQueue<String> upper = new PriorityQueue<>(Collections.reverseOrder());
        PriorityQueue<String> lower = new PriorityQueue<>(Collections.reverseOrder());
        
        for(int i=0; i<s.length(); i++) {
            char c = s.charAt(i);
            if(c>='a' && c<='z') { lower.add(String.valueOf(c)); }
            else { upper.add(String.valueOf(c)); }
        }
        
        while(!lower.isEmpty()) { answer += lower.poll(); }
        while(!upper.isEmpty()) { answer += upper.poll(); }
        
        return answer;
    }
}