Maximum Depth of Binary Tree

Given the root of a binary tree (represented as level-order array where null indicates absence), 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: tree = [3,9,20,null,null,15,7]
Output: 3

Constraints

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