735. Asteroid Collision

README.md

@@ -43,6 +43,7 @@ continually updating 😃.
 
 ### Stack
 * [155. Min Stack](src/0155_min_stack/min_stack.go)
+* [735. Asteroid Collision](src/0735_asteroid_collision/ac.go)
 
 ### String
 * [3. Longest Substring Without Repeating Characters](./src/0003_longest_substring_without_repeating_characters/longest_substring_without_repeating_characters.go)   *`sliding window;`*  *`hash table`*

src/0735_asteroid_collision/ac.go

@@ -0,0 +1,46 @@
+/*
+735. Asteroid Collision
+https://leetcode.com/problems/asteroid-collision/
+
+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.
+
+Note:
+The length of asteroids will be at most 10000.
+Each asteroid will be a non-zero integer in the range [-1000, 1000]..
+*/
+// time: 2019-01-14
+
+package ac
+
+// stack
+// time complexity:  O(N), where NN is the number of asteroids. Our stack pushes and pops each asteroid at most once.
+// space complexity: O(N), the size of stack.
+func asteroidCollision(asteroids []int) []int {
+	stack := make([]int, 0)
+
+	for _, asteroid := range asteroids {
+		flag := true
+		for len(stack) > 0 && asteroid < 0 && stack[len(stack)-1] > 0 {
+			if stack[len(stack)-1] == -asteroid {
+				stack = stack[:len(stack)-1]
+			} else if stack[len(stack)-1] < -asteroid {
+				stack = stack[:len(stack)-1]
+				continue
+			}
+			flag = false
+			break
+		}
+		if flag {
+			stack = append(stack, asteroid)
+		}
+	}
+	return stack
+}

src/0735_asteroid_collision/ac_test.go

@@ -0,0 +1,30 @@
+package ac
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestAsteroidCollision(t *testing.T) {
+	testCases := [][]int{
+		{5, 10, -5},
+		{-2, 2, -1, -2},
+		{-2, -1, 1, 2},
+		{8, -8},
+		{10, 2, -5},
+	}
+
+	expected := [][]int{
+		{5, 10},
+		{-2},
+		{-2, -1, 1, 2},
+		{},
+		{10},
+	}
+
+	for index, asteroids := range testCases {
+		if res := asteroidCollision(asteroids); !reflect.DeepEqual(res, expected[index]) {
+			t.Errorf("expected %v, got %v", expected[index], res)
+		}
+	}
+}

src/README.md

@@ -102,5 +102,6 @@
 |0713|[713. Subarray Product Less Than K](0713_subarray_product_less_than_k/spltk.go)|Medium|*`sliding window`*|
 |0717|[717. 1-bit and 2-bit Characters](0717_1_bit_and_2_bit_characters/1bitand2bitc.go)|Easy||
 |0728|[Self Dividing Numbers](./0728_self_dividing_numbers/self_dividing_numbers.go)|Easy||
+|0735|[735. Asteroid Collision](0735_asteroid_collision/ac.go)|Medium|*`stack`*|
 |0747|[Largest Number At Least Twice of Others](./0747_largest_number_at_least_twice_of_others/largest_number_at_least_twice_of_others.go)|Easy||
 |0872|[872. Leaf-Similar Trees](0872_leaf_similar_trees/leaf_similar_trees.go)|Easy|*`binary tree`*|