class Solution {
public:
    int trap(vector<int>& height) {
        int l = 0, r = height.size() - 1;
        int lmax = 0,rmax = 0,ans = 0;
        while(l < r){
            lmax = max(lmax, height[l]);
            rmax = max(rmax, height[r]);
            if(lmax <= rmax){
                ans += lmax - height[l];
                l++; 
            }else{
                ans += rmax - height[r];
                r--;
            }
        }
        return ans;
    }
};