Find the maximum depth of a binary tree (the number of nodes along the longest path from the root to the farthest leaf node).

The tree is represented as a nested object: { val: 3, left: { val: 9, left: null, right: null }, right: { val: 20, left: null, right: null } }

Example:

Input: root = [3,9,20,null,null,15,7]
        3
       / \
      9  20
        /  \
       15   7
Output: 3