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]]
Example 3:
- Input: root = []
- Output: []
Constraints:
- The number of nodes in the tree is in the range [0, 2000].
- -1000 <= Node.val <= 1000