LeetCode-590-N叉树的后序遍历

原题链接：https://leetcode-cn.com/problems/n-ary-tree-postorder-traversal/

给定一个 N 叉树，返回其节点值的后序遍历。
例如，给定一个 3叉树 :

image.png

解题思路：

1. 递归略；
2. 迭代法，仿照N叉树的前序遍历，前序遍历的顺序是【中、左、右】，只需实现【中、右、左】再反转得【左、右、中】即是后续遍历的结果；

Python3代码：

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

class Solution:
    # 递归
    def postorder(self, root: 'Node') -> List[int]:
        if not root: return []
        ans = []
        if root.children:
            for child in root.children:
                ans.extend(self.postorder(child))
        ans.extend([root.val])
        return ans

"""
# Definition for a Node.
class Node:
    def __init__(self, val=None, children=None):
        self.val = val
        self.children = children
"""
class Solution:
    # 迭代
    def postorder(self, root: 'Node') -> List[int]:
        if not root: return None
        stack = [root]
        ans = []
        while stack:
            node = stack.pop()
            ans.append(node.val)
            stack.extend(node.children)
        return ans[::-1]