Given the root of a binary tree, invert the tree by swapping the left and right children of all nodes and return its root.
Example 1:
Input: root = [4, 2, 7, 1, 3, 6, 9]
Output: [4, 7, 2, 9, 6, 3, 1]
Explanation: The tree is mirrored — every left child is swapped with its corresponding right child.
Example 2:
Input: root = [2, 1, 3]
Output: [2, 3, 1]
Example 3:
Input: root = []
Output: []
[0, 100]-100 <= Node.val <= 100