Merge pull request #41 from mlkr/master

239 两个更快的解法 O(n)

Author: aQuaYi

Algorithms/0239.sliding-window-maximum/sliding-window-maximum.go

@@ -1,28 +1,78 @@
 package problem0239
 
 func maxSlidingWindow(nums []int, k int) []int {
-	// 题目里面说好了 1 <= k 的
-	// testcase 里面还是出现了 k == 0
-	if k == 0 {
-		return nil
-	}
+	res := make([]int, 0, len(nums)-k+1)
+	window := make([]int, 0, k)
+	for i, v := range nums {
+		// 控制 window 窗口大小
+		if i > k-1 && window[0] == i-k {
+			window = window[1:]
+		}
 
-	// res 的长度是可计算的
-	res := make([]int, len(nums)-k+1)
-	for i := 0; i+k <= len(nums); i++ {
-		res[i] = maxOf(nums[i : i+k])
+		// window 保持递减
+		j := len(window) - 1
+		for ; j >= 0; j-- {
+			if nums[window[j]] > v {
+				break
+			}
+		}
+		window = window[:j+1]
+		window = append(window, i)
+
+		// 获取 window 最大值
+		if i >= k-1 {
+			res = append(res, nums[window[0]])
+		}
 	}
 
 	return res
 }
 
-// 获取局部的最大值
-func maxOf(nums []int) int {
-	max := nums[0]
-	for i := 1; i < len(nums); i++ {
-		if max < nums[i] {
-			max = nums[i]
+// 解法二
+// 参看 https://leetcode.com/problems/sliding-window-maximum/discuss/65881/O(n)-solution-in-Java-with-two-simple-pass-in-the-array
+// 例子
+// nums 1 3 3 -3 5 5 6 7
+// k = 3
+// left  1 3 3 | -3 5 5 | 6 7
+// right 3 3 -1|  5 5 3 | 7 7
+// res   3 3 5    5 6 7
+func maxSlidingWindow2(nums []int, k int) []int {
+	size := len(nums)
+	if size <= 1 {
+		return nums
+	}
+
+	left, right := make([]int, size), make([]int, size)
+	left[0], right[size-1] = nums[0], nums[size-1]
+
+	for i := 1; i < size; i++ {
+		if i%k == 0 {
+			left[i] = nums[i]
+		} else {
+			left[i] = max(nums[i], left[i-1])
+		}
+
+		j := size - 1 - i
+		if (j+1)%k == 0 {
+			right[j] = nums[j]
+		} else {
+			right[j] = max(nums[j], right[j+1])
 		}
 	}
-	return max
+
+	res := make([]int, size-k+1)
+	for i := 0; i <= size-k; i++ {
+		res[i] = max(right[i], left[i+k-1])
+	}
+
+	return res
+
+}
+
+func max(a, b int) int {
+	if a > b {
+		return a
+	}
+
+	return b
 }

Algorithms/0239.sliding-window-maximum/sliding-window-maximum_test.go

@@ -17,7 +17,7 @@ var tcs = []struct {
 	{
 		[]int{},
 		0,
-		nil,
+		[]int{},
 	},
 
 	{
@@ -38,10 +38,27 @@ func Test_maxSlidingWindow(t *testing.T) {
 	}
 }
 
+func Test_maxSlidingWindow2(t *testing.T) {
+	ast := assert.New(t)
+
+	for _, tc := range tcs {
+		fmt.Printf("~~%v~~\n", tc)
+		ast.Equal(tc.ans, maxSlidingWindow2(tc.nums, tc.k), "输入:%v", tc)
+	}
+}
+
 func Benchmark_maxSlidingWindow(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		for _, tc := range tcs {
 			maxSlidingWindow(tc.nums, tc.k)
 		}
 	}
 }
+
+func Benchmark_maxSlidingWindow2(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		for _, tc := range tcs {
+			maxSlidingWindow2(tc.nums, tc.k)
+		}
+	}
+}