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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.
Input: root = [3,9,20,null,null,15,7] Output: 3
Input: root = [1,null,2] Output: 2
Input: root =  Output: 0
Input: root =  Output: 1
- The number of nodes in the tree is in the range
-100 <= Node.val <= 100