42. 接雨水

• 题目链接：42. 接雨水
• 双指针

class Solution(object):def trap(self, height):""":type height: List[int]:rtype: int"""lh = [0]*len(height)rh = [0]*len(height)lh[0]=height[0]for i in range(1, len(height)):lh[i] = max(height[i],lh[i-1])rh[-1]=height[-1]for j in range(len(height)-2, -1, -1):rh[j] = max(height[j], rh[j+1])result = 0for k in range(len(height)):result += min(lh[k], rh[k])-height[k]return result

• 单调栈

class Solution(object):def trap(self, height):""":type height: List[int]:rtype: int"""result = 0stack =[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]]:sub = height[stack[-1]]stack.pop()if stack:h = min(height[stack[-1]], height[i])-subw = i-stack[-1]-1result+=w*hstack.append(i)return result

84.  柱状图中最大的矩形

• 题目链接：84.  柱状图中最大的矩形
• 为什么要首尾加零：防止单调递增递减序列的情况

class Solution(object):def largestRectangleArea(self, heights):""":type heights: List[int]:rtype: int"""heights.insert(0,0)heights.append(0)result = 0stack = [0]for i in range(1, len(heights)):if heights[i]>heights[stack[-1]]:stack.append(i)elif heights[i]==heights[stack[-1]]:stack.pop()stack.append(i)else:while stack and heights[i]<heights[stack[-1]]:mid_index = stack[-1]stack.pop()if stack:w = i-stack[-1]-1h = heights[mid_index]result = max(result, w*h)stack.append(i)return result