Trapping Rain Water

class Solution {public:    int trap(vector<int>& height)     {        if(height.empty())            return 0;        int left=0;        int right=height.size()-1;        int size=0;        int leftWall=height[left];        int rightWall=height[right];        while(left < right)        {            if(leftWall<=rightWall)            {                ++left;                if(height[left]<leftWall)                    size+=leftWall-height[left];                else                    leftWall=height[left];            }            else            {                --right;                if(height[right]<rightWall)                    size+=rightWall-height[right];                else                    rightWall=height[right];            }        }                return size;            }};