题解 | #图片整理#

import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    public static void main(String[] args) {

        Map<Character, Integer> map =  new HashMap<>();
        Scanner in = new Scanner(System.in);
        String s = "";
        // 注意 hasNext 和 hasNextLine 的区别
        while (in.hasNext()) { // 注意 while 处理多个 case
            s = in.next();
        }

        int len = s.length();

        for (int i = 0; i < len; i++) {
            map.put(s.charAt(i), map.getOrDefault(s.charAt(i), 0) + 1);
        }
        for (char i = '0'; i <= '9'; i++) {
            Integer count = map.get(i);
            if (count != null) {
                for (int j = 0; j < count; j++) {
                    System.out.print(i);
                }
            }
        }
        for (char i = 'A'; i <= 'Z'; i++) {
            Integer count = map.get(i);
            if (count != null) {
                for (int j = 0; j < count; j++) {
                    System.out.print(i);
                }
            }
        }
        for (char i = 'a'; i <= 'z'; i++) {
            Integer count = map.get(i);
            if (count != null) {
                for (int j = 0; j < count; j++) {
                    System.out.print(i);
                }
            }
        }

    }
}

算是取巧吗？