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: []

  Constraints:

The number of nodes in the tree is in the range [0, 100].
-100 <= Node.val <= 100