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......
        int i = str.length();
        StringBuilder sb = new StringBuilder();
        while(i >= 3){
            sb.insert(0,str.substring(i-3,i));
            sb.insert(0,",");
            i-=3;
        }
        if(i>0){
            sb.insert(0,str.substring(0,i));
        }
        System.out.println(sb.toString());
    }
}