Invert Binary Tree

Given the `root` of a binary tree, invert the tree, and return its root. **Example 1:** ``` Input: root = [4,2,7,1,3,6,9] Output: [4,7,2,9,6,3,1] ``` **Example 2:** ``` Input: root = [2,1,3] Output: [2,3,1] ``` **Example 3:** ``` Input: root = [] Output: [] ```