Given the `root` of a binary tree, return the level order traversal of its nodes' values (i.e., from left to right, level by level).
**Example 1:**
```
Input: root = [3,9,20,null,null,15,7]
Output: [[3],[9,20],[15,7]]
```
**Example 2:**
```
Input: root = [1]
Output: [[1]]
```