这棵树有几片叶子

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 leaves(node):
    if node is None:
        return 0
    if node.left is None and node.right is None:
        return 1
    return leaves(node.left) + leaves(node.right)

print(leaves(build()))