589. N-ary Tree Preorder Traversal

# Definition for a Node.
class Node(object):
    def __init__(self, val, children):
        self.val = val
        self.children = children


class Solution(object):

    def preOrderHelp(self, root, path):

        if root is None:
            return
        path.append(root.val)
        children = root.children
        for ch in children:
            self.preOrderHelp(ch, path)

    def preorder(self, root):
        """
        :type root: Node
        :rtype: List[int]
        """
        re_li = []
        self.preOrderHelp(root, re_li)
        return re_li