这个题其实很简单，只要保持需要读取值的那一层的节点就可以了。
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] ]
link:{sourcedir}/_0102_BinaryTreeLevelOrderTraversal.java[role=include]