Leetcode 刷题笔记1 单调栈part02

leetcode 42 接雨水

本题用双指针法更为浅显易懂

双指针法：

class Solution:
    def trap(self, height: List[int]) -> int:
        leftheight, rightheight = [0] * len(height), [0] * len(height)
        ans = 0
        leftheight[0] = height[0]
        for i in range(1, len(height)):
            leftheight[i] = max(height[i], leftheight[i - 1])
        rightheight[-1] = height[-1]
        for i in range(len(height) - 2, -1, -1):
            rightheight[i] = max(height[i], rightheight[i + 1])
        for i in range(len(height)):
            total = min(leftheight[i], rightheight[i]) - height[i]
            ans += total
        return ans

单调栈：

class Solution:
    def trap(self, height: List[int]) -> int:
        stack = [0]
        ans = 0
        for i in range(1, len(height)):
            if height[i] < height[stack[-1]]:
                stack.append(i)
            elif height[i] == height[stack[-1]]:
                stack.pop()
                stack.append(i)
            else:
                while stack and height[i] > height[stack[-1]]:
                    mid_height = height[stack[-1]]
                    stack.pop()
                    if stack:
                        right_height = height[i]
                        left_height = height[stack[-1]]
                        h = min(right_height, left_height) - mid_height
                        w = i - stack[-1] - 1
                        ans += h * w
                stack.append(i)
        return ans

leetcode 84 柱状图的最大的矩形

class Solution:
    def largestRectangleArea(self, heights: List[int]) -> int:
        heights.insert(0, 0)
        heights.append(0)
        stack = [0]
        ans = 0
        for i in range(1, len(heights)):
            while stack and heights[i] < heights[stack[-1]]:
                mid_height = heights[stack[-1]]
                stack.pop()
                if stack:
                    area = (i - stack[-1] - 1) * mid_height
                    ans = max(area, ans)
            stack.append(i)
        return ans

双指针法：
 

class Solution:
    def largestRectangleArea(self, heights: List[int]) -> int:
        n = len(heights)
        min_left_index = [0] * n
        min_right_index = [0] * n
        min_left_index[0] = -1
        result = 0
        for i in range(1, n):
            temp = i - 1
            while temp >= 0 and heights[temp] >= heights[i]:
                temp = min_left_index[temp]
            min_left_index[i] = temp
        min_right_index[n - 1] = n
        for i in range(n - 2, -1, -1):
            temp = i + 1
            while temp < n and heights[temp] >= heights[i]:
                temp = min_right_index[temp]
            min_right_index[i] = temp
        for i in range(len(heights)):
            area = heights[i] * (min_right_index[i] - min_left_index[i] -1)
            result = max(result, area)
        return result