Given the root of a binary tree, return its maximum depth.

A binary tree's maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

Example 1:

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

Example 2:

Input:  1 null 2
Output: 2

Constraints:

• The number of nodes in the tree is in the range [0, 10⁴]
• -100 ≤ Node.val ≤ 100

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

Output format: Integer — the maximum depth.