Invert Binary Tree

The problem. Given the root of a binary tree, invert it — mirror it left to right — and return its root.

Example

Input:  root = [4, 2, 7, 1, 3, 6, 9]
Output: [4, 7, 2, 9, 6, 3, 1]

Input:  root = [2, 1, 3]     Output: [2, 3, 1]
Input:  root = []            Output: []

Look at a single node in the mirror image: its two children have traded places — and each child's subtree is itself mirrored. So: swap the children of every node.