Maximum Depth of a Binary Tree

Given the root of a binary tree, return how tall it is. Start at the root and walk down to a node with no children (a leaf). Count the nodes on the longest such walk. That count is the tree's maximum depth.

• An empty tree (root is null) has depth 0.
• A tree that is just a root has depth 1.
• The tree can have up to 1,000 nodes, and it may lean heavily to one side.

Each node is a TreeNode with val, left and right. The starter already exports this class.