LeetCode-145. 二叉树的后序遍历

给定一个二叉树，返回它的 后序 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 postorderTraversal(self, root: TreeNode) -> List[int]:
        self.nums = []
        def postorder(root):
            if not root:
                return
            postorder(root.left)
            postorder(root.right)
            self.nums.append(root.val)
        
        postorder(root)
        return self.nums