Trapping_Rain_Water.cpp

 int trap(vector<int>& height) {
        int n = height.size();
         int left=0; int right=n-1;
        int ans=0;
        int maxl=0, maxr=0;
        while(left<=right){
            if(height[left]<=height[right]){
                if(height[left]>=maxl) maxl=height[left];
                else ans+=maxl-height[left];
                left++;
            }
            else{
                if(height[right]>=maxr) maxr= height[right];
                else ans+=maxr-height[right];
                right--;
            }
        }
        return ans;
    }