asteroid-collision.py

# Time:  O(n)
# Space: O(n)

# We are given an array asteroids of integers representing asteroids in a row.
#
# For each asteroid, the absolute value represents its size,
# and the sign represents its direction (positive meaning right, negative meaning left).
# Each asteroid moves at the same speed.
#
# Find out the state of the asteroids after all collisions.
# If two asteroids meet, the smaller one will explode. If both are the same size, both will explode.
# Two asteroids moving in the same direction will never meet.
#
# Example 1:
# Input: 
# asteroids = [5, 10, -5]
# Output: [5, 10]
# Explanation: 
# The 10 and -5 collide resulting in 10.  The 5 and 10 never collide.
# Example 2:
# Input: 
# asteroids = [8, -8]
# Output: []
# Explanation: 
# The 8 and -8 collide exploding each other.
# Example 3:
# Input: 
# asteroids = [10, 2, -5]
# Output: [10]
# Explanation: 
# The 2 and -5 collide resulting in -5.  The 10 and -5 collide resulting in 10.
# Example 4:
# Input: 
# asteroids = [-2, -1, 1, 2]
# Output: [-2, -1, 1, 2]
# Explanation: 
# The -2 and -1 are moving left, while the 1 and 2 are moving right.
# Asteroids moving the same direction never meet, so no asteroids will meet each other.
#
# Note:
# - The length of asteroids will be at most 10000.
# - Each asteroid will be a non-zero integer in the range [-1000, 1000].

class Solution(object):
    def asteroidCollision(self, asteroids):
        """
        :type asteroids: List[int]
        :rtype: List[int]
        """
        result = []
        for asteroid in asteroids:
            while result and asteroid < 0 < result[-1]:
                if result[-1] < -asteroid:
                    result.pop()
                    continue
                elif result[-1] == -asteroid:
                    result.pop()
                break
            else:
                result.append(asteroid)
        return result