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Maximum Depth of Binary Tree
EasyData StructuresDescription
Given the root of a binary tree represented as a level-order array (with `null` for missing nodes), 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.
For example, `[3, 9, 20, null, null, 15, 7]` represents:
```
3
/ \
9 20
/ \
15 7
```
The maximum depth is 3.
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 number of nodes in the tree is in the range [0, 10^4].
- -100 <= Node.val <= 100
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