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();
        StringBuilder sen=new StringBuilder(str);
        for(int i=sen.length()-3;i>=0;)
        {
           sen.insert(i,',');
           i=i-3;
        }
        
        System.out.print(sen);
    }
}