题解 | #字符统计#

import java.util.Scanner;
import java.util.HashMap;
import java.util.Collections;
import java.util.ArrayList;
import java.util.Comparator;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        while (sc.hasNextLine()) {
            String str = sc.nextLine();
            HashMap<Character, Integer> map = new HashMap<>();
            //把 字符-出现次数 放入HashMap中
            for (char ch : str.toCharArray()) {
                map.put(ch, map.getOrDefault(ch, 0) + 1);
            }
            // 再把字符放入ArrayList中，自定义比较器进行sort
            ArrayList<Character> chList = new ArrayList<>(map.keySet());
            Collections.sort(chList, new Comparator<Character>() {
                public int compare(Character c1, Character c2) {
                    int num = map.get(c2)-map.get(c1);
                    int num2 = num==0?c1 - c2:num;
                    return num2;
                }
            });
            //输出排序后的字符
            for (Character ch : chList) {
                System.out.print(ch.toString());
            }
            System.out.println();
        }
        sc.close();
    }
}