如何修改我当前的二叉搜索树以创建一个空树？

Question

so far my code for creating a binary search tree is fine for what I'm trying to accomplish. However I would like for the new function to create an empty tree but my whole code as far as I can understand is based on a root value called when instancing a new object. How would I go about changing the code for new to return an empty BST instead of a tree with one root value as base? A relevant issue is also that the size attribute needs to start at 1 and I would like for it to start at 0. Thank you in advance.

Below is all the code linked.

class BinaryTree:

    def __init__(self, root):
        self._root = root
        self._left = None
        self._right = None
        self._size = 1

    def add(self, root):
        if self._root:
            if root < self._root:
                if self._left is None:
                    self._left = BinaryTree(root)
                    self._size += 1
                else:
                    self._left.add(root)
                    self._size += 1
            elif root > self._root:
                if self._right is None:
                    self._right = BinaryTree(root)
                    self._size += 1
                else:
                    self._right.add(root)
                    self._size += 1
        else:
            self._root = root

    def string(self):
        if self._left:
            self._left.string()
        print(self._root),
        if self._right:
            self._right.string()

    def len(self):
        return self._size



def new(root):
    return BinaryTree(root)

Answer 1

I've mark added/changed lines with # changed comment:

class BinaryTree:

    def __init__(self, root=None):  # changed
        self._root = root
        self._left = None
        self._right = None
        self._size = 1 if root is not None else 0  # changed

    def add(self, root):
        if self._root:
            if root < self._root:
                if self._left is None:
                    self._left = BinaryTree(root)
                    self._size += 1
                else:
                    self._left.add(root)
                    self._size += 1
            elif root > self._root:
                if self._right is None:
                    self._right = BinaryTree(root)
                    self._size += 1
                else:
                    self._right.add(root)
                    self._size += 1
        else:
            self._root = root
            self._size = 1  # changed

    def string(self):
        if self._left:
            self._left.string()
        print(self._root)  # changed, removed ","
        if self._right:
            self._right.string()

    def len(self):
        return self._size


def new(root=None):
    return BinaryTree(root)

Check also magic methods __str__ and __len__ they could help. Method add migth returns self which allow to add elements one by one in chain like that add(1).add(10).add(100) .