给定Python中二叉树节点的定义如下，下列选项中是二叉树前序遍历的正确递归实现的是？

第31721题单选题

二叉树节点定义代码：

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

def preorder(root: TreeNode) -> None:
    if not root:
        return
    print(root.val)
    preorder(root.left)
    preorder(root.right)

def preorder(root: TreeNode) -> None:
    if not root:
        return
    preorder(root.left)
    print(root.val)
    preorder(root.right)

def preorder(root: TreeNode) -> None:
    if not root:
        return
    preorder(root.left)
    preorder(root.right)
    print(root.val)

def preorder(root: TreeNode) -> None:
    print(root.val)
    preorder(root.left)
    preorder(root.right)

程序运行统计

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知识点：

Python-二叉树的递归实现

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