Return the maximum depth of a binary tree.
Example 1
Input: root = [1,null,2]
Output: 2
Explanation: The depth of the right subtree is 2.
Output: 2
Explanation: The depth of the right subtree is 2.
Example 2
Input: root = [3,9,20,null,null,15,7]
Output: 3
Explanation: The maximum depth is 3 levels.
Output: 3
Explanation: The maximum depth is 3 levels.