题目链接：429. N 叉树的层序遍历
广度优先搜索。
"""
# Definition for a Node.
class Node:
def __init__(self, val=None, children=None):
self.val = val
self.children = children
"""
class Solution:
def levelOrder(self, root: 'Node') -> List[List[int]]:
if root is None:
return []
ret, nodes = [], [root]
while nodes:
next_nodes, nums = [], []
for node in nodes:
nums.append(node.val)
next_nodes.extend(node.children)
ret.append(nums)
nodes = next_nodes
return ret