A26933. 请将下列 Python 实现的深度优先搜索（DFS）代码补充完整，横线处应填入（ ）。from typing import List, Optional class TreeNode: def __init__(self, val=0, left=None, right=None): self.val = val self.left = left self.right = right def d…

请将下列 Python 实现的深度优先搜索（DFS）代码补充完整，横线处应填入（    ）。

from typing import List, Optional

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

def dfs_preorder(root: Optional[TreeNode], result: List[int]) -> None:
	if root is None:
		return
	_____________________________