"""

# Definition for a Node.

class Node(object):

    def __init__(self, val, children):

        self.val = val

        self.children = children

"""

class Solution(object):

    def preorder(self, root):

        """

        :type root: Node

        :rtype: List[int]

        """

        ret[] #初始化结果数组

        if root is None:return [] #树空的情况返回[]，所以这里不能return None

        #递归时候把该节点加入ret。 root相当于上层的x

        #有个问题，每次递归会把ret置空，导致最后ret=[]

        ret.append(root.val)

        for x in root.children:

            #这一步很重要，递归时把每一层的ret元素联合起来

            ret.extend(self.preorder(x))

        return ret