leetcode 145. Binary Tree Postor

Given a binary tree, return the postorder traversal of its nodes' values.
Example:

Input: [1,null,2,3]
   1
    \
     2
    /
   3

Output: [3,2,1]

Follow up: Recursive solution is trivial, could you do it iteratively?

这个和leetcode144类似，是利用递归求后序遍历。

python代码：

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def postorderTraversal(self, root: TreeNode) -> List[int]:
        nums = []
        if not root:
            return nums
        nums.extend(self.postorderTraversal(root.left))
        nums.extend(self.postorderTraversal(root.right))
        nums.append(root.val)
        return nums