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:
[0, 100].-100 <= Node.val <= 100type TreeNode struct {
	Val   int
	Left  *TreeNode
	Right *TreeNode
}
/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func invertTree(root *TreeNode) *TreeNode {
	if root == nil {
		return nil
	}
	temp := root.Left
	root.Left = invertTree(root.Right)
	root.Right = invertTree(temp)
	return root
}