【leetcode】589. N-ary Tree Preorder Traversal

class Solution(object):
    def preorder(self, root):
        """
        :type root: Node
        :rtype: List[int]
        """
        if root == None:
            return []
        res = []
        stack = [root]
        while len(stack) > 0:
            node = stack.pop(0)
            res.append(node.val)
            for i in node.children[::-1]:
                stack.insert(0,i)
        return res