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: root = [3,9,20,null,null,15,7]
• Output: 3

Example 2:

• Input: root = [1,null,2]
• Output: 2

Constraints:

• The number of nodes in the tree is in the range [0, 10^4].
• -100 <= Node.val <= 100