C++:

 1 class Solution {
 2 public:
 3     int trap(vector<int>& height) {
 4         int left = 0 ;
 5         int right = height.size() - 1;
 6         int res = 0 ;
 7         int maxLeft = 0 , maxRight = 0 ;
 8         while(left <= right){
 9             if (height[left] <= height[right]){
10                 if (height[left] >= maxLeft){
11                     maxLeft = height[left] ;
12                 }else{
13                     res += maxLeft - height[left] ;
14                 }
15                 left++ ;
16             }else{
17                 if (height[right] >= maxRight){
18                     maxRight = height[right] ;
19                 }else{
20                     res += maxRight - height[right] ;
21                 }
22                 right-- ;
23             }
24         }
25         return res ;
26     }
27 };