Java-我怎么知道Math.rand（）或Random将生成哪个数字？

Question

Here is an algorithm problem that I am trying to solve(It's written in Korean.): https://www.acmicpc.net/problem/10944

If I simply translate the problem, I need to generate a number between 1 and 10,000. The resulting number must be the same as the random number which is generated by the server(a referee)

Is this possible? Can I guess what the system server would generate and Can I match it with my program's result?

I figured out that there are two kinds of way to make a random number in Java.

1. Random
2. Math.random()

And I can manipulate(or predefine) the result with Random. And I just think it might be possible to solve the problem.

For example:

import java.io.IOException;
import java.util.Random;

public class Solution {
    public static void main(String[] args) throws IOException {
        Random rnd = new Random(1);
        System.out.println(rnd.nextInt(10000));

        rnd = new Random(1);
        System.out.println(rnd.nextInt(10000));
    }
}

It shows the same result because the parameter of Random is the same. And I tried with Random. But I couldn't solve that problem.

Like this:

import java.io.IOException;
import java.util.Random;

public class Solution {
    public static void main(String[] args) throws IOException {
        Random rnd = new Random(1);
        System.out.println(rnd.nextInt(10000));
    }
}

And as I remember, Math.random() is generated by system time. But I can't find any information for that. If that's true and it may be possible to solve the problem. Because I can program with the system time and the result will always correspond.

Answer 1

Your question has the same basis as the google authenticator implementation. Most RSA tokens or 2factor authentications use this method to authenticate the user. As mentioned in the comments use a common seed to solve the problem