# 735. Asteroid Collision 行星碰撞

@TOC

## # 题目描述

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]..

## # 解题方法

### # 栈

``````class Solution(object):
def asteroidCollision(self, asteroids):
"""
:type asteroids: List[int]
:rtype: List[int]
"""
stack = []
for ast in asteroids:
while stack and ast < 0 and stack[-1] >= 0:
pre = stack.pop()
if ast == -pre:
ast = None
break
elif -ast < pre:
ast = pre
if ast != None:
stack.append(ast)
return stack
``````

``````class Solution {
public:
vector<int> asteroidCollision(vector<int>& asteroids) {
vector<int> s;
for (int a : asteroids) {
bool ispush = true;
while (!s.empty() && a < 0 && s.back() > 0) {
int t = s.back();
if (abs(a) > abs(t)) {
s.pop_back();
} else if (abs(a) == abs(t)) {
s.pop_back();
ispush = false;
break;
} else {
ispush = false;
break;
}
}
if (ispush)
s.push_back(a);
}
return s;
}
};
``````

## # 日期

2018 年 7 月 17 日 —— 连天大雨，这种情况很少见，但是很舒服 2018 年 12 月 26 日 —— 转眼就周三了，一万年太久，只争朝夕