N-ary Tree Preorder Traversal Python Solution

lcpy gen -n 589   # by problem number
lcpy gen -s n_ary_tree_preorder_traversal   # by problem name

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

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: [1,2,3,6,7,11,14,4,8,12,5,9,13,10]

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 preorder(self, root: NaryNode | None) -> list[int]:
        if root is None:
            return []
        result: list[int] = []
        stack: list[NaryNode] = [root]
        while stack:
            node = stack.pop()
            result.append(node.val)
            stack.extend(reversed(node.children))
        return result