42

class Solution {
public:
    int trap(vector<int>& height) {
        int left, right, ans, left_max, right_max;
        left = 0; right = height.size()-1;
        left_max = right_max = ans = 0;

        while(left < right){
            if(height[left] < height[right]){
                height[left] >= left_max ? left_max = height[left] : ans += (left_max - height[left]);
                left++;
            }
            else{
                height[right] >= right_max ? right_max = height[right] : ans += (right_max - height[right]);
                right--;
            }
        }
        return ans;
    }
};