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