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 str1 = new StringBuilder(str);
        for (int i=str.length()-3; i>=0; i=i-3) {
            str1.insert(i, ",");
        }
        System.out.println(str1);

    }
}