Invert Binary Tree
Difficulty: Easy
Category: DSA
Topics: Tree, Depth-First Search, Breadth-First Search, Binary Tree
Asked at: Microsoft, Amazon, Google, Facebook, eBay, Paypal
Given the `root` of a binary tree, invert the tree, and return _its root_.
**Example 1:**
**Input:** root = [4,2,7,1,3,6,9]
**Output:** [4,7,2,9,6,3,1]
```
**Example 2:**
**Input:** root = [2,1,3]
**Output:** [2,3,1]
```
**Example 3:**
**Input:** root = []
**Output:** []
```
**Constraints:**
- The number of nodes in the tree is in the range `[0, 100]`.
- `-100 <= Node.val <= 100`