Just do the simple preorder traversal. The only difference between binary tree preorder traversal is that we need to traversal all the children of each node.
"""
# 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]:
order = []
self._helper(root, order)
return order
def _helper(self, node, order):
if node is None:
return
order.append(node.val)
for child in node.children:
self._helper(child, order)