42. Trapping Rain Water

Posted 2020-09-08 为了更优秀的你，加油！

Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it is able to trap after raining.

For example, 
Given [0,1,0,2,1,0,1,3,2,1,2,1], return 6.

The above elevation map is represented by array [0,1,0,2,1,0,1,3,2,1,2,1]. In this case, 6 units of rain water (blue section) are being trapped. Thanks Marcos for contributing this image!

class Solution {
public:
    //夹逼准则。每次让低的那一侧移动。直到相遇
    int trap(vector<int>& height) {
        int low=0,high=height.size()-1,res=0;
        int leftmax=0,rightmax=0;
        while(low<=high){
            if(height[low]<height[high]){
                leftmax=max(leftmax,height[low]);
                res+=leftmax-height[low];
                low++;
            }
            else {
                rightmax=max(rightmax,height[high]);
                res+=rightmax-height[high];
                high--;
            }
        }
        return res;
    }
};