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:
3Example 2
Input:
root = [1,null,2]Output:
2Constraints
- •
The number of nodes in the tree is in the range [0, 10^4]. - •
-100 <= Node.val <= 100
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