Python 3:二进制搜索树，size函数不返回节点数

class Node:
    def __init__(self, value):
        self.value = value
        self.left = None
        self.right = None

class BinarySearchTree:
    def __init__(self):
        self.root = None
        self.size = 0

    def insert(self, value):
        if self.root is None:
            self.root = Node(value)
        else:
            self._insert_recursive(self.root, value)
        self.size += 1

    def _insert_recursive(self, node, value):
        if value < node.value:
            if node.left is None:
                node.left = Node(value)
            else:
                self._insert_recursive(node.left, value)
        else:
            if node.right is None:
                node.right = Node(value)
            else:
                self._insert_recursive(node.right, value)

    def get_size(self):
        return self.size