Invert Binary Tree

Given the root of a binary tree, invert the tree and return its root. Inverting a binary tree means swapping the left and right children of every node in the tree. The transformation may be performed in place.

The number of nodes is between 0 and 100, and each node value is between -100 and 100.

Examples:

• Input tree [4,2,7,1,3,6,9] becomes [4,7,2,9,6,3,1].
• Input tree [2,1,3] becomes [2,3,1].
• An empty tree [] remains [].

Return None when the input tree is empty.