这棵树有多高

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

def height(node):
    if node is None:
        return 0
    a = height(node.left)
    b = height(node.right)
    return 1 + (a if a > b else b)

print(height(build()))