solved https://leetcode-cn.com/problems/sliding-window-maximum/

Author: psc0606

Diff for: README.md

@@ -89,6 +89,7 @@ Aha, algorithm by go. See Chinese [README.md](./README_cn.md)
 * [Rotate Array](src/array/rotate-array.go) [M] [I]
 * [Group Anagrams](src/array/group-anagrams.go) [M] [I]
 * [K-th Largest Element In An Array](src/array/kth-largest-element-in-an-array.go) [M] [I]
+* [Sliding Window Maximum](src/array/sliding-window-maximum.go) [M] [I]
 
 ---
 ### List
@@ -105,6 +106,10 @@ Aha, algorithm by go. See Chinese [README.md](./README_cn.md)
 * [Valid Parentheses](src/stack/valid-parentheses.go) [E] [I]
 * [Decode String](src/stack/decode-string.go) [M]
 
+---
+### Queue
+* [Sliding Window Maximum](src/array/sliding-window-maximum.go) [M] [I]
+
 ---
 ### Dynamic Programming And Greedy
 * [Climbing Stairs](src/optimalization/climbing-stairs.go) [E] [I]

Diff for: README_cn.md

@@ -89,6 +89,7 @@
 * [旋转数组](src/array/rotate-array.go) [M] [I]
 * [字母异位词分组](src/array/group-anagrams.go) [M] [I]
 * [数组中的第K个最大元素](src/array/kth-largest-element-in-an-array.go) [M] [I]
+* [滑动窗口中最大值](src/array/sliding-window-maximum.go) [M] [I]
 
 ---
 ### 列表
@@ -105,6 +106,10 @@
 * [有效的括号](src/stack/valid-parentheses.go) [E] [I]
 * [字符串解码](src/stack/decode-string.go) [M]
 
+---
+### 队列
+* [滑动窗口中最大值](src/array/sliding-window-maximum.go) [M] [I]
+
 ---
 ### 动态规划和贪心
 * [爬楼梯](src/optimalization/climbing-stairs.go) [E] [I]

Diff for: src/array/sliding-window-maximum.go

@@ -0,0 +1,66 @@
+package array
+
+// https://leetcode-cn.com/problems/sliding-window-maximum/
+
+// Use two stack, but time complexity is O(n^2)
+func maxSlidingWindow(nums []int, k int) []int {
+	n := len(nums)
+	var stack, help []int
+	var ret []int
+	for i := 0; i < n; i++ {
+		// stack store max value, the max value is always on the top.
+		// the element in stack sort by: min(bottom) -> max(top)
+		for len(stack) > 0 && stack[len(stack)-1] > nums[i] {
+			ele := stack[len(stack)-1]
+			stack = stack[:len(stack)-1]
+			help = append(help, ele)
+		}
+		stack = append(stack, nums[i])
+		for len(help) > 0 { // restore the popped elements
+			stack = append(stack, help[len(help)-1])
+			help = help[:len(help)-1]
+		}
+
+		if i-k >= 0 { // find the element then delete it
+			for stack[len(stack)-1] != nums[i-k] {
+				ele := stack[len(stack)-1]
+				stack = stack[:len(stack)-1]
+				help = append(help, ele)
+			}
+			stack = stack[:len(stack)-1]
+			for len(help) > 0 { // restore
+				stack = append(stack, help[len(help)-1])
+				help = help[:len(help)-1]
+			}
+		}
+		if len(stack) >= k {
+			// top of stack is the window maximum
+			ret = append(ret, stack[len(stack)-1])
+		}
+	}
+	return ret
+}
+
+// The above solution problem: we do some useless stack push and pop, there is no need help stack.
+func maxSlidingWindowOptimizeWithDequeue(nums []int, k int) []int {
+	n := len(nums)
+	var dequeue []int
+	var ret []int
+	for i := 0; i < n; i++ {
+		// dequeue sort the elements in window from max(left) to min(right).
+		// When we enqueue the current nums[i], we will discard all elements small than nums[i], because the current window's
+		// maximum elements must in [max, ..., nums[i]].
+		for len(dequeue) > 0 && dequeue[len(dequeue)-1] < nums[i] {
+			dequeue = dequeue[:len(dequeue)-1] // pop end
+		}
+		dequeue = append(dequeue, nums[i])
+		if i-k+1 >= 0 {
+			// if dequeue[0] == nums[i-k]
+			if i-k >= 0 && dequeue[0] == nums[i-k] {
+				dequeue = dequeue[1:] // pop top
+			}
+			ret = append(ret, dequeue[0])
+		}
+	}
+	return ret
+}

Diff for: src/array/sliding-window-maximum_test.go

@@ -0,0 +1,70 @@
+package array
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestMaxSlidingWindow(t *testing.T) {
+	nums := []int{1, 3, -1, -3, 5, 3, 6, 7}
+	actual, expected := maxSlidingWindow(nums, 3), []int{3, 3, 5, 5, 6, 7}
+	if !reflect.DeepEqual(actual, expected) {
+		t.Errorf("expected is [%v], actual is [%v]", expected, actual)
+	}
+
+	nums = []int{1, 3, 2, 4, 5, 3, 6, 7}
+	actual, expected = maxSlidingWindow(nums, 3), []int{3, 4, 5, 5, 6, 7}
+	if !reflect.DeepEqual(actual, expected) {
+		t.Errorf("expected is [%v], actual is [%v]", expected, actual)
+	}
+
+	nums = []int{1}
+	actual, expected = maxSlidingWindow(nums, 1), []int{1}
+	if !reflect.DeepEqual(actual, expected) {
+		t.Errorf("expected is [%v], actual is [%v]", expected, actual)
+	}
+}
+
+func TestMaxSlidingWindowOptimizeWithDequeue(t *testing.T) {
+	nums := []int{1, 3, -1, -3, 5, 3, 6, 7}
+	actual, expected := maxSlidingWindowOptimizeWithDequeue(nums, 3), []int{3, 3, 5, 5, 6, 7}
+	if !reflect.DeepEqual(actual, expected) {
+		t.Errorf("expected is [%v], actual is [%v]", expected, actual)
+	}
+
+	nums = []int{1, 3, 2, 4, 5, 3, 6, 7}
+	actual, expected = maxSlidingWindowOptimizeWithDequeue(nums, 3), []int{3, 4, 5, 5, 6, 7}
+	if !reflect.DeepEqual(actual, expected) {
+		t.Errorf("expected is [%v], actual is [%v]", expected, actual)
+	}
+
+	nums = []int{1}
+	actual, expected = maxSlidingWindowOptimizeWithDequeue(nums, 1), []int{1}
+	if !reflect.DeepEqual(actual, expected) {
+		t.Errorf("expected is [%v], actual is [%v]", expected, actual)
+	}
+
+	nums = []int{1, -1}
+	actual, expected = maxSlidingWindowOptimizeWithDequeue(nums, 1), []int{1, -1}
+	if !reflect.DeepEqual(actual, expected) {
+		t.Errorf("expected is [%v], actual is [%v]", expected, actual)
+	}
+
+	nums = []int{1, -1, 1}
+	actual, expected = maxSlidingWindowOptimizeWithDequeue(nums, 2), []int{1, 1}
+	if !reflect.DeepEqual(actual, expected) {
+		t.Errorf("expected is [%v], actual is [%v]", expected, actual)
+	}
+
+	nums = []int{1, -1, -1}
+	actual, expected = maxSlidingWindowOptimizeWithDequeue(nums, 1), []int{1, -1, -1}
+	if !reflect.DeepEqual(actual, expected) {
+		t.Errorf("expected is [%v], actual is [%v]", expected, actual)
+	}
+
+	nums = []int{1, -1, -1}
+	actual, expected = maxSlidingWindowOptimizeWithDequeue(nums, 2), []int{1, -1}
+	if !reflect.DeepEqual(actual, expected) {
+		t.Errorf("expected is [%v], actual is [%v]", expected, actual)
+	}
+}