如何将二叉树追加到另一个二叉树

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

def append_tree(target_tree, append_tree):
    if target_tree is None:
        return append_tree
    if append_tree is None:
        return target_tree

    target_tree.left = append_tree(target_tree.left, append_tree.left)
    target_tree.right = append_tree(target_tree.right, append_tree.right)

    return target_tree

# 创建两个二叉树
tree1 = TreeNode(1)
tree1.left = TreeNode(2)
tree1.right = TreeNode(3)

tree2 = TreeNode(4)
tree2.left = TreeNode(5)
tree2.right = TreeNode(6)

# 将tree2追加到tree1
append_tree(tree1, tree2)

     1
    / \
   4   3
  / \
 5   6