42. Trapping Rain Water

C++:

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