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],
Example 1:
3
/ \
9 20
/ \
15 7
[
[3],
[9,20],
[15,7]
]
Tags: Math, String
给定一个二叉树，返回其按层次遍历的节点值。
```go
### 思路2
> 思路2
```go
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