Asteroid Collision (Swift Solution)

This repository contains a Swift implementation of the Asteroid Collision problem —
a popular stack-based algorithm question often seen in coding interviews and platforms like LeetCode.

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##Problem Description

We are given an array asteroids of integers representing asteroids in a row.
The indices represent their relative position in space.

• Each asteroid’s absolute value represents its size.
• The sign represents its direction:
  • Positive → moving right
  • Negative → moving left

When two asteroids meet:

• The smaller one will explode.
• If both are the same size, both explode.
• Asteroids moving in the same direction never collide.

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Example 1 Input:
[5, 10, -5]
Output:
[5, 10]
Explanation:
10 and -5 collide → 10 survives.
5 and 10 never collide.

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Example 2 Input:
[8, -8]
Output:
[]
Explanation:
Both explode.

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Example 3 Input:
[10, 2, -5]
Output:
[10]
Explanation:
2 and -5 collide → -5 survives.
10 and -5 collide → 10 survives.

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Swift Code

func asteroidCollision(_ asteroids: [Int]) -> [Int] {
    var stack = [Int]()
    
    for asteroid in asteroids {
        var current = asteroid
        var destroyed = false
        
        while let last = stack.last, last > 0, current < 0 {
            if abs(last) < abs(current) {
                stack.removeLast()
                continue
            } else if abs(last) == abs(current) {
                stack.removeLast()
                destroyed = true
                break
            } else {
                destroyed = true
                break
            }
        }
        
        if !destroyed {
            stack.append(current)
        }
    }
    
    return stack
}


| Complexity | Description                                            | 
| **Time**   | O(n) — Each asteroid is pushed/popped at most once     |
| **Space**  | O(n) — Stack may store all asteroids in the worst case |