Invert Binary Tree

Problem

Given the root of a binary tree, invert the tree, and return its root. Inverting a binary tree means that each left child becomes a right child, and every right child becomes a left child.

Examples

invertTree([1,2,3])
// [1,3,2]

invertTree([1,2])
// [1,null,2]

invertTree([1,2,3,4,5,6,7])
// [1,3,2,7,6,5,4]
invertTree([1])
// [1]