# [102. Binary Tree Level Order Traversal](https://leetcode.com/problems/binary-tree-level-order-traversal/)

## 题目


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],

```c
    3
   / \
  9  20
    /  \
   15   7
```

return its level order traversal as:

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

## 题目大意

按层序从上到下遍历一颗树。

## 解题思路

用一个队列即可实现。