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 newstr = new StringBuilder(str);//将固定的字符串变为可变的字符串
        for(int i = str.length()-3; i>0 ;i-=3){
            newstr.insert(i,",");
        }
        System.out.print(newstr.toString());

    }
}