public class Solution
extends Object
226 - Invert Binary Tree\.
Easy
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`