leetcode【中等】144、二叉树的前序遍历

思路：参考

# 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: TreeNode) -> List[int]:
        def preorder(root,res):
            if not root:
                return
            res.append(root.val)
            preorder(root.left,res)
            preorder(root.right,res)

        res=[]
        preorder(root,res)
        return res