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();
        StringBuffer string = new StringBuffer(str);
        int count=0;
        for(int i =string.length();i>0;i--)
        {
            if(++count==3)
            {
                string.insert(i-1,",");
                count=0;
            }
        }
        System.out.println(string);
    }
}