题解 | #字符统计#

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        while (in.hasNextLine()) {
            String input = in.nextLine();
            Map<Character, Integer> map = new TreeMap<>();
            for (int i = 0; i < input.length(); i++) {
                char c = input.charAt(i);
                if (map.containsKey(c)) {
                    map.put(c, map.get(c) + 1);
                } else {
                    map.put(c, 0);
                }
            }
            Integer[][] intArray = new Integer[map.size()][2];
            char[] chars = new char[map.size()];
            int index = 0;
            for (Map.Entry<Character, Integer> entry : map.entrySet()) {
                chars[index] = entry.getKey();
                intArray[index][0] = entry.getValue();
                intArray[index][1] = index;
                index++;
            }
            Arrays.sort(intArray, (o1, o2) -> o2[0] - o1[0]);
            for (int i = 0; i < intArray.length; i++) {
                System.out.print(chars[intArray[i][1]]);
            }
        }
    }
}