Invert a Binary Tree

Question

## Problem

Invert a binary tree (mirror it) and return the root.

## Example
```
[4,2,7,1,3,6,9] -> [4,7,2,9,6,3,1]
```

## Constraints
- 0 ≤ nodes ≤ 100

## Approach
Recursively swap children, or iterative BFS/DFS.