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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.
Example 1
Input: root = [3,9,20,null,null,15,7]
Output: 3
Explanation: The longest path from the root is 3 → 20 → 15 (or 3 → 20 → 7). It has 3 nodes.
Example 2
Input: root = [1,null,2]
Output: 2
Explanation: The only path is 1 → 2, which has 2 nodes.
Example 3
Input: root = []
Output: 0
Explanation: The tree is empty, so its depth is 0.
The depth of a tree is one more than the deeper of its two subtrees. Recurse to the leaves (depth 0 for a missing child) and combine on the way back up.
Run checks all cases above. Submit evaluates all test cases.