AVL树中“空”节点数量的O复杂度是多少？

class TreeNode:
    def __init__(self, key):
        self.key = key
        self.left = None
        self.right = None
        self.height = 1

def get_height(node):
    if not node:
        return 0
    return node.height

def update_height(node):
    if not node:
        return 0
    node.height = 1 + max(get_height(node.left), get_height(node.right))

def count_empty_nodes(root):
    if not root:
        return 0
    left_empty = count_empty_nodes(root.left)
    right_empty = count_empty_nodes(root.right)
    return left_empty + right_empty + (1 if not root.left else 0) + (1 if not root.right else 0)

# 示例用法
root = TreeNode(10)
root.left = TreeNode(5)
root.right = TreeNode(15)
root.left.left = TreeNode(3)
root.left.right = TreeNode(7)

print("空节点数量:", count_empty_nodes(root))