这棵树一共几层

运行下面这段程序：

class TNode:
    def __init__(self, name):
        self.name = name
        self.left = None
        self.right = None

def build():
    root = TNode("总部")
    root.left = TNode("华北")
    root.right = TNode("华南")
    root.left.left = TNode("北京")
    root.left.right = TNode("天津")
    root.right.right = TNode("广州")
    return root

q = [build()]
lv = 0
while len(q) > 0:
    size = len(q)
    for _ in range(size):
        node = q.pop(0)
        if node.left is not None:
            q.append(node.left)
        if node.right is not None:
            q.append(node.right)
    lv += 1
print(lv)