Maximum Depth of Binary Tree Easy

Given the root of a binary tree (TreeNode with val, left, right; null for empty), return its maximum depth: the number of nodes along the longest path from the root down to a leaf.

Examples

• [3,9,20,null,null,15,7] -> 3
• [] -> 0

Define a function named maxDepth in JavaScript. TreeNode (val, left, right) is available.