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 stb = new StringBuilder(str);
        StringBuilder str1=stb.reverse();
        
        for (int i = 3; i < str1.length(); i=i+4) {
            str1.insert(i,",");
        }
        str1.reverse();
        System.out.println(str1);

    }
}