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:  3 9 20 null null 15 7
Output:
3
9 20
15 7

Example 2:

Input:  1
Output:
1

Constraints:

• The number of nodes in the tree is in the range [0, 2000]
• -1000 ≤ Node.val ≤ 1000

Input format: BFS level-order, space-separated, null for missing nodes.

Output format: One line per level, values space-separated.