Maximum Depth of Binary Tree

You get the root of a binary tree. Return the depth of the tree.

The depth counts the nodes on the longest path from the root down to a leaf. A leaf is a node with no children. An empty tree has a depth of 0.

The trees below are listed level by level, top to bottom and left to right. A missing child is written as None.

Example 1:

      8
     / \
    4   -2
   /
  5
   \
    1

Input: root = [8,4,-2,5,None,None,None,None,1]
Output: 4
Explanation: The longest way down is 8 → 4 → 5 → 1.
It has 4 nodes.
The way to the other leaf, 8 → -2, has only 2 nodes.

Example 2:

Input: root = [6]
Output: 1
Explanation: The root is also the only leaf.

Example 3:

Input: root = []
Output: 0
Explanation: The tree is empty.

Constraints:

• 0 to 10,000 nodes.
• Each value is from -100 to 100.
• The tree is at most 400 levels deep.