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Maximum Depth of Binary Tree
Difficulty: Easy
Category: DSA
Topics: Tree, Depth-First Search, Breadth-First Search, Binary Tree
Asked at: LinkedIn, Amazon, Intel
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
```
**Example 2:**
**Input:** root = [1,null,2]
**Output:** 2
```
**Constraints:**
- The number of nodes in the tree is in the range `[0, 104]`.
- `-100 <= Node.val <= 100`
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