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 sb1 = new StringBuilder(str).reverse();
        StringBuilder sb2 = new StringBuilder();
        int p = 0;
        for (int i = 0; i < sb1.length(); i++) {
            if (p % 3 == 0 && p != 0) {
                sb2.append(",");
            }
            sb2.append(sb1.charAt(i));
            p++;
        }
        System.out.println(sb2.reverse().toString());

    }
}