题解 | #动态字符串#

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String str = scanner.next();

        //write your code here......
        StringBuilder s = new StringBuilder(str);
        for (int i = str.length() - 3; i >= 0; i-=3) {
            s.insert(i,",");
        }
        System.out.println(s);

    }
}