The second question of the 13th CCF certification

1. It means that there are several small balls on a line, each moving in a certain direction at the same speed
2. Reverse after collision
3. At most, there will be two small balls colliding at the same time. I won't talk about it if there's proof
4. Ask the position of all the balls at a certain time

Code:

import java.util.Scanner;
public class second
{
    public static void main(String[] args)
    {
        Scanner scanner=new Scanner(System.in);
        String string=scanner.nextLine();
        String sd[]=string.split(" ");
        int sum[]=transfer(sd);
        int ball_number=sum[0];
        int linelength=sum[1];
        int time=sum[2];
        String strball=scanner.nextLine();
        int balls[]=transfer(strball.split(" "));
//deal
        int position[]=new int [ball_number];
        int direction[]=new int [ball_number];

        boolean bian[]=new boolean[ball_number];
        for (int i = 0; i < bian.length; i++) {
            bian[i]=false;
        }
        //direction
        for (int i = 0; i < direction.length; i++) {
            direction[i]=1;
        }
        //position
        for (int i = 0; i < position.length; i++) {
             position[i]=balls[i];
        }
        int now=0;
        while(now<time){
            now++; 
            // A new round can change
            for (int i = 0; i < bian.length; i++) {
                bian[i]=false;
            }

            for (int i = 0; i < position.length; i++) {

                //Disguised form
                if (position[i]==linelength||position[i]==0) {
//                  System.out.println("hit the wall" + position[i]);
                    direction[i]=-direction[i];
                }
                //Colliding in disguise
                for (int j = 0; j < direction.length; j++) {
                    if (!bian[j]&&i!=j&&position[i]==position[j]){
//                      System.out.println("Main.main()"+position[i]+position[j]);
                        direction[i]=-direction[i];
                        direction[j]=-direction[j];
                        bian[j]=true;
                        bian[i]=true;
                    }
                }
            }
            for (int i = 0; i < position.length; i++) {
                position[i]+=direction[i];
            }
        }
        out(position);
    }
    static void out (int a[]){
        for (int i = 0; i < a.length; i++) {
            System.out.print(a[i]+" ");
        }
        System.out.println();
    }
    static int[] transfer(String str[]){
        int a[]=new int [str.length];
                for (int i = 0; i < str.length; i++) {
            a[i]=Integer.parseInt(str[i]);              
        }
         return a;
    }
}

The thought code has given almost, the final result is also correct, I hope to help you.
Use a while loop to record the state of all the balls in every second, and then get the state of all the balls in t second step by step.

Because a computer is a state machine in essence. It calculates the current state according to the state of the last second, and so on