leetcode102 Binary Tree Level Order Traversal

 """
 Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).
 For example:
 Given binary tree [3,9,20,null,null,15,7],
     3
    / \
   9  20
     /  \
    15   7
 return its level order traversal as:
 [
   [3],
   [9,20],
   [15,7]
 ]

 """
 class TreeNode:
     def __init__(self, x):
         self.val = x
         self.left = None
         self.right = None

 class Solution:
     def levelOrder(self, root):
         if root ==  None:    #树为空，直接返回[]
             return []
         queue = [root]
         cur = []
         res = []
         while queue:
             cur = [x.val if x else None for x in queue] #存当前层的value
             newqueue = []                   #newqueue用来遍历下一层使用
             for x in queue:
                 if x.left:                  #bug 如果不判断，结果包含[None]
                     newqueue.append(x.left)
                 if x.right:
                     newqueue.append(x.right)
             queue = newqueue
             res.append(cur)                 #将当前[]append到最后结果
         return res
 """
 本题与leetcode101相似，用BFS可解
 """