Maximum Depth of Binary Tree

Given the root of a binary tree, 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.

Examples

Example 1:

Input: root = [3, 9, 20, null, null, 15, 7]

Output: 3

Explanation: The longest path is 3 → 20 → 15 (or 3 → 20 → 7), which has 3 nodes.

Example 2:

Input: root = [1, null, 2]

Output: 2

Constraints

• The number of nodes in the tree is in the range [0, 10⁴]
• -100 <= Node.val <= 100