Trapping Rain Water

function trap(height) {
    let total = 0;
    let l = 0, r = height.length - 1;
    let lmax = 0, rmax = height[r];

    while(l < r) {
        if(height[l] <= height[r]) {
            if(height[l] < lmax) {
                total += lmax - height[l];
            } else {
                lmax = height[l];
            }
            l++;
        } else {
            if(height[r] < rmax) {
                total += rmax - height[r];
            } else {
                rmax = height[r];
            }
            r--;
        }
    }
    return total;
};