590. N 叉树的后序遍历

"""
# Definition for a Node.
class Node:
    def __init__(self, val=None, children=None):
        self.val = val
        self.children = children
"""

class Solution:
    def postorder(self, root: 'Node') -> List[int]:
        if root is None:
            return []

        stack, ret = [root], []
        while stack:
            node = stack.pop()
            ret.append(node.val)
            for child in (node.children or []):
                stack.append(child)

        return ret[::-1]