Calculates filled gaps between pikes

volume.py

def calculate_volume(terrain):
    if len(terrain) < 3: return 0
    lmax = 0
    rmax = 0
    left = 0
    right = len(terrain) - 1
    volume = 0;
    while left < right:
        if terrain[left] > lmax:
            lmax = terrain[left]
        if terrain[right] > rmax:
            rmax = terrain[right]

        if lmax > rmax:
            volume += rmax - terrain[right]
            right -= 1
        else:
            volume += lmax - terrain[left] 
            left += 1

    return volume