def height(root):
    queue = deque([root])
    height = -1

    while queue:
        level_len = len(queue)
        height += 1

        for _ in range(level_len):
            current = queue.popleft()
            if current.left:
                queue.append(current.left)
            if current.right:
                queue.append(current.right)
    
    return height