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