N-ary tree preorder traversal

We will traverse the tree with recursive DFS, as it is preorder traversal, we first append the root to our result and then traverse all the children.

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

class Solution:
    def preorder(self, root: 'Node') -> List[int]:
        res = []

        def traverse(root, res):
            if not root:
                return
            res.append(root.val)
            for child in root.children:
                traverse(child, res)

        traverse(root, res)
        return res

Time Complexity: O(n)
Space Complexity: O(n)