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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]]
思路：没什么好说的，和107题：107. Binary Tree Level Order Traversal II 比较像，都是用的队列和二叉树广度优先遍历，一个是正序输出一个是逆序输出。
107题的解题过程：http://blog.csdn.net/kangbin825/article/details/53966578
public List
> levelOrder(TreeNode root) { List
> list = new ArrayList
>(); Queue queue = new LinkedList (); if(root==null) { return list; } queue.offer(root); while(!queue.isEmpty()) { List levelList = new ArrayList (); int num = queue.size(); for(int i=0;i