Given the root of a binary tree, return its maximum depth: the number of nodes along the longest path from the root down to a leaf.
Trees are written level by level, with null for a missing child. [3, 9, 20, null, null, 15, 7] is a root 3 with children 9 and 20, where 20 has children 15 and 7.
Examples
Example 1
- Input
- root = [3, 9, 20, null, null, 15, 7]
- Output
- 3
Example 2
- Input
- root = [1, null, 2]
- Output
- 2
Example 3
- Input
- root = []
- Output
- 0
Constraints
- The tree has between
0and10^4nodes. -100 <= Node.val <= 100