42. Trapping Rain Water

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:
    int trap(vector<int>& height) {
        if(height.size() < 3) return 0;
        int res = 0;
        int left = 0;
        int right = height.size() - 1;
        int leftMax = height[0];
        int rightMax = height[right];

        while(left != right){
            if(leftMax < rightMax){
                left++;
                if(height[left]<leftMax){
                    res +=leftMax - height[left];
                }else{
                    leftMax = height[left];
                }
            }else{
                right--;
                if(height[right] < rightMax){
                    res += rightMax- height[right];
                }else{
                    rightMax = height[right];
                }
            }
        }
        return res;
    }
};