https://leetcode.com/problems/binary-tree-level-order-traversal/
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