Problem - Invert Binary Tree

Given the root of a binary tree, invert the tree and return its root. Inverting means swapping left and right children at every node. The result is a mirror image.

Example: $[4,2,7,1,3,6,9]$ becomes $[4,7,2,9,6,3,1]$. This is famously the problem that inspired a Homebrew developer to tweet about failing a Google interview.

Despite its simplicity, it tests whether you understand tree recursion. Constraints: up to $100$ nodes, values from $-100$ to $100$.