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.next();
        HashMap<Character, Integer> map = new HashMap<>();
        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);
            }
        }
        List<Ch> list = new ArrayList<>();
        for (Character key : map.keySet()) {
            list.add(new Ch(key, map.get(key)));
        }
        Collections.sort(list, new Comparator<Ch>() {
            public int compare(Ch p, Ch q) {
                if (p.order > q.order) {
                    return -1;
                } else if (p.order < q.order) {
                    return 1;
                } else {
                    return p.c - q.c;
                }
            }
        });
        for (Ch p : list) {
            System.out.print(p.c);
        }
    }
}
class Ch {
    char c;
    int order;
    public Ch() {}
    public Ch(char c, int order) {
        this.c = c;
        this.order = order;
    }
}