Create Binary Tree Level Order Traversal 

两个队列，一个出一个存，当出的空了以后，存的开始输出到list,同时存到出队列当中，依次类推。

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,#,#,15,7},
    3
   / \
  9  20
    /  \
   15   7
return its level order traversal as:
[
  [3],
  [9,20],
  [15,7]
]