class Solution:
    def preorderTraversal(self , root: TreeNode) -> List[int]:
        if not root:
            return []
        
        mid = [root.val]
        left = self.preorderTraversal(root.left)
        right = self.preorderTraversal(root.right)
        
        return mid + left + right

递归是最方便的

没有石蒜第六集看我要死了