Binary Tree — Maximum Depth

solve(levelOrder) receives a binary tree serialized level by level, with
null for a missing child. A helper that builds the tree is already provided.
Return the tree's maximum depth: the number of nodes on the longest path
from the root down to a leaf. An empty tree has depth 0.

This is the "hello world" of tree recursion: the answer for a node is built from
the answers for its two subtrees.

Examples

solve([3, 9, 20, null, null, 15, 7])  → 3
solve([1, null, 2])                   → 2
solve([])                             → 0