代码随想录算法训练营Day60 | 单调栈(3/3) LeetCode 84.柱状图中最大的矩形

今天就是训练的最后一天了，刷满60天，感觉很有成就感！

84. Largest Rectangle in Histogram

Given an array of integers heights representing the histogram's bar height where the width of each bar is 1, return the area of the largest rectangle in the histogram.

用单调栈的思路来解这道题，本题是要找每个柱子左右两边第一个小于该柱子的柱子，所以从栈头（元素从栈头弹出）到栈底的顺序应该是从大到小的顺序。栈顶和栈顶的下一个元素以及要入栈的三个元素组成了我们要求最大面积的高度和宽度。

class Solution:
    def largestRectangleArea(self, heights: List[int]) -> int:
        heights.insert(0, 0)
        heights.append(0)
        stack = [0]
        result = 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 = width * height
                    area = (i - stack[-1] - 1) * mid_height
                    result = max(area, result)
            stack.append(i)
        return result