题解 | #二叉树的前序遍历#

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


class Solution:
    def preorderTraversal(self, root: TreeNode) -> List[int]:
        if not root:
            return []
        res = []

        def dfs(node: TreeNode):
            res.append(node.val)
            if node.left:
                dfs(node.left)
            if node.right:
                dfs(node.right)

        dfs(root)
        return res