LeetCode: N-ary Tree Postorder Traversal

## https://code.dennyzhang.com/n-ary-tree-postorder-traversal
## Basic Ideas:
## Complexity: Time O(n), Space O(n)
"""
# Definition for a Node.
class Node(object):
    def __init__(self, val, children):
        self.val = val
        self.children = children
"""
class Solution(object):
    def postorder(self, root):
        """
        :type root: Node
        :rtype: List[int]
        """
        if root == None: return []
        res = []
        for node in root.children:
            res += self.postorder(node)
        res.append(root.val)
        return res