讲的好的视频讲解
【很难想象这up刷题的精神状态 Leetcode42. 接雨水】
https://www.bilibili.com/video/BV1MC411n7Af/?share_source=copy_web&vd_source=afbacdc02063c57e7a2ef256a4db9d2a
rm是right max的意思，lm是left max的意思
时间复杂度：$O(n)$
空间复杂度:$S(1)$

class Solution {
public:int trap(vector<int>& height) {int res = 0 , l = 0,r = height.size() - 1 ,lm = 0 ,rm = 0;while(l < r){lm = max(lm,height[l]);rm = max(rm , height[r]);if(lm < rm)res += lm - height[l++];//柱子宽度是1，面积计算过程省略了*1的步骤elseres += rm - height[r--];}return res;}
};