题解 | #字符统计#

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String s = sc.nextLine();

        TreeMap<Character, Integer> map = new TreeMap<>();
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            if (map.containsKey(c)) map.put(c, map.get(c) + 1);
            else map.put(c, 1);
        }

        int max = 0;
        for (Integer value : map.values()) {
            if (value > max) max = value;
        }

        while (max > 0) {
            Set<Character> keySet = map.keySet();
            for (Character c : keySet) {
                if (map.get(c) == max) System.out.print(c);
            }
            max--;
        }
    }
}