题解 | 添加逗号

import java.util.*;
public class Main{
    public static void main(String[] args){
        Scanner in = new Scanner(System.in);
        String s = in.next();
        StringBuilder s1 = new StringBuilder(s),res = new StringBuilder();
        s1.reverse();
        for(int i=0;i<s1.length();i++){
            res.append(s1.charAt(i));
            if((i+1)%3==0&&i!=s1.length()-1){
                res.append(",");
            }
        }
        res.reverse();
        s = res.toString();
        System.out.println(s);
    }
}