AsteroidCollision.java

package Stacks;

import java.util.Stack;

public class AsteroidCollision {

    public static int[] asteroidCollision(int[] asteroids) {
        int n = asteroids.length;
        Stack<Integer> s = new Stack<>();

        for (int asteroid : asteroids) {
            if (asteroid > 0 || s.isEmpty()) {
                s.push(asteroid);
            } else {
                while (!s.isEmpty() && s.peek() > 0 && s.peek() < Math.abs(asteroid)) {
                    s.pop();
                }
                if (!s.isEmpty() && s.peek() == Math.abs(asteroid)) {
                    s.pop();
                } else {
                    if (s.isEmpty() || s.peek() < 0) {
                        s.push(asteroid);
                    }
                }
            }
        }

        int[] res = new int[s.size()];
        for (int i = s.size() - 1; i >= 0; i--) {
            res[i] = s.pop();
        }
        return res;
    }

    public static void main(String[] args) {
        int[] asteroids = {5, 10, -5};
        int[] ans = asteroidCollision(asteroids);
        for (int num : ans) {
            System.out.print(num + " ");
        }
    }
}