LeetCode-144. 二叉树的前序遍历

给你二叉树的根节点 root ，返回它节点值的 前序 遍历。

# 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]:
        self.nums = []
        def postorder(root):
            if not root:
                return
            self.nums.append(root.val)
            postorder(root.left)
            postorder(root.right)
            
        postorder(root)
        return self.nums