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
Example 3
Input: root = []
Output: 0
Constraints
•The number of nodes in the tree is in the range [0, 10⁴].