Java Coding Test Course, Fast Forward with Time Machine

In this article, we will cover how to prepare for coding tests using the Java language and go through the process of solving an actual algorithm problem. The topic is ‘Traveling Fast with a Time Machine’. Through this problem, we will learn graph search techniques and algorithm optimization, as well as how to analyze and solve problems.

Problem Description

You are on a journey traveling back and forth in time using a time machine. The time machine can go back in time under the following conditions:

• Move one step forward after 1 second from the current position
• Move one step backward after 2 seconds from the current position

Your goal is to arrive at the X coordinate after Z seconds. Determine the minimum time required to use the time machine.

Input Format

Integer X, Z (-10^6 ≤ X ≤ 10^6, 0 ≤ Z ≤ 10^6)

Output Format

Minimum time to use the time machine. Print -1 if it's impossible

Examples

Problem Solving Process

To solve this problem, it is effective to understand the conditions for using the time machine to reach the desired position X and apply the BFS (Breadth-First Search) algorithm based on this. BFS is advantageous for finding the shortest path under given conditions.

Step 1: Define the State

We will use current position and elapsed time to represent each state of the time machine.

Step 2: Define the Movement Rules

• Move one step forward: current position + 1
• Move one step backward: current position - 1

Step 3: Implement BFS

import java.util.*;

public class TimeMachine {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int X = sc.nextInt();
        int Z = sc.nextInt();
        System.out.println(minimumTime(X, Z));
    }
    
    public static int minimumTime(int target, int z) {
        Queue queue = new LinkedList<>();
        Set visited = new HashSet<>();
        queue.add(new int[]{0, 0});  // {current position, elapsed time}
        
        while (!queue.isEmpty()) {
            int[] current = queue.poll();
            int position = current[0];
            int time = current[1];

            if (time > z) continue; // Ignore if it exceeds Z seconds
            if (position == target && time == z) return time;

            // Move one step forward
            if (!visited.contains(position + 1)) {
                queue.add(new int[]{position + 1, time + 1});
                visited.add(position + 1);
            }

            // Move one step backward
            if (!visited.contains(position - 1)) {
                queue.add(new int[]{position - 1, time + 2});
                visited.add(position - 1);
            }
        }
        return -1;
    }
}

Step 4: Testing and Optimization

After writing the above code, test it with various input values. Ensure that it correctly handles impossible cases as well. Additionally, consider ways to optimize memory usage and execution time.

Conclusion

In this lesson, we learned how to cleverly solve algorithm problems using Java. Through the ‘Traveling Fast with a Time Machine’ problem, we experienced the basics of the BFS algorithm and the effectiveness of state space exploration. Continue solving various problems to build your skills.