N-ary Tree Postorder Traversal Python Solution

lcpy gen -n 590   # by problem number
lcpy gen -s n_ary_tree_postorder_traversal   # by problem name

Input: root = [1,null,3,2,4,null,5,6]
Output: [5,6,3,2,4,1]

Input: root = [1,null,2,3,4,5,null,null,6,7,null,8,null,9,10,null,null,11,null,12,null,13,null,null,14]
Output: [2,6,14,11,7,3,12,8,4,13,9,10,5,1]

from __future__ import annotations


class NaryNode:
    def __init__(self, val: int = 0, children: list[NaryNode] | None = None) -> None:
        self.val = val
        self.children = children if children is not None else []


class Solution:
    # Time: O(n)
    # Space: O(n)
    def postorder(self, root: NaryNode | None) -> list[int]:
        result: list[int] = []
        if root is None:
            return result
        stack = [root]
        while stack:
            node = stack.pop()
            result.append(node.val)
            stack.extend(node.children)
        # Reverse preorder (children visited right-to-left) equals postorder.
        result.reverse()
        return result