Maximum Depth of Binary Tree

Given the root of a binary tree, return its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

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Example 1:

    3
   / \
  9  20
    /  \
   15   7

Input: root = [3,9,20,null,null,15,7]
Output: 3
Explanation: The longest root-to-leaf path has 3 nodes: 3 -> 20 -> 15 (or 3 -> 20 -> 7).

Example 2:

Input: root = [1,null,2]
Output: 2
Explanation: The only leaf is node 2; reached via a path of length 2 from the root.

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Constraints

• The number of nodes in the tree is in the range [0, 10^4].
• -100 <= Node.val <= 100