Given the root of a binary tree, return its maximum depth. Maximum depth is the number of nodes along the longest path from root to a leaf.
Example: A tree with root 3, left child 9, right child 20, and 20's children 15 and 7 has maximum depth 3. You're solving a foundational problem for recursive tree thinking.
The answer for a node depends on the answers for its children. Constraints: up to nodes, values from to .