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 / \ 920 / \ 157

return its level order traversal as:

[ [3], [9,20], [15,7] ]

按层序从上到下遍历一颗树。

用一个队列即可实现。