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
_____________________________result.append(root.val)
dfs_preorder(root.left, result)
dfs_preorder(root.right, result)result.append(root.val)
dfs_preorder(root.left, result)result.append(root.val)
dfs_preorder(root.right, result)dfs_preorder(root.left, result)
dfs_preorder(root.right, result)