现有Python实现的二叉树前序DFS代码如下，请补全横线处内容：

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

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