42. 接雨水

接雨水这道题目是 面试中特别高频的一道题，也是单调栈 应用的题目，大家好好做做。

建议是掌握 双指针 和单调栈，因为在面试中 写出单调栈可能 有点难度，但双指针思路更直接一些。

在时间紧张的情况有，能写出双指针法也是不错的，然后可以和面试官在慢慢讨论如何优化。

代码随想录

class Solution {     public int trap(int[] height) {         int total = 0;         for (int index = 0; index < height.length; index++) {             if (index == 0 || index == height.length - 1) continue;              int rightMax = height[index];              int leftMax = height[index];              for (int right = index + 1; right < height.length; right++) {                 rightMax = Math.max(rightMax, height[right]);             }             for (int left = index - 1; left >= 0; left--) {                 leftMax = Math.max(leftMax, height[left]);             }             int waterHeight = Math.min(leftMax, rightMax) - height[index];             if (waterHeight > 0) total += waterHeight;         }         return total;     } }

1. 柱状图中最大的矩形

有了之前单调栈的铺垫，这道题目就不难了。

class Solution {     public int largestRectangleArea(int[] heights) {         int[] extendedHeights = new int[heights.length + 2];         System.arraycopy(heights, 0, extendedHeights, 1, heights.length);         extendedHeights[0] = 0;         extendedHeights[heights.length + 1] = 0;          Stack stack = new Stack<>();         stack.push(0);          int maxArea = 0;         for (int i = 1; i < extendedHeights.length; i++) {             while (!stack.isEmpty() && extendedHeights[i] < extendedHeights[stack.peek()]) {                 int topIndex = stack.pop();                 int width = i - stack.isEmpty() ? 0 : i - stack.peek() - 1;                 int height = extendedHeights[topIndex];                 maxArea = Math.max(maxArea, width * height);             }             stack.push(i);         }         return maxArea;     } }

代码随想录