144. 二叉树的前序遍历（放松AC）

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def preorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        res = []
        def dfs(root):
            if not root:
                return res
            res.append(root.val)
            if root.left:dfs(root.left)
            if root.right:dfs(root.right)
        dfs(root)
        return res