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 line = scanner.next();
        StringBuilder sb = new StringBuilder(line);
        for (int i = line.length() - 3; i >= 0; i -= 3) {
            sb.insert(i, ',');
        }
        System.out.println(sb);
    }
}