Invert Binary Tree

 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

/**

 * Definition for a binary tree node.

 * public class TreeNode {

 *     int val;

 *     TreeNode left;

 *     TreeNode right;

 *     TreeNode() {}

 *     TreeNode(int val) { this.val = val; }

 *     TreeNode(int val, TreeNode left, TreeNode right) {

 *         this.val = val;

 *         this.left = left;

 *         this.right = right;

 *     }

 * }

 */

class Solution {

    public TreeNode invertTree(TreeNode root) {

         if (root == null) return null;

        // Swap left and right recursively

        TreeNode left = invertTree(root.left);

        TreeNode right = invertTree(root.right);

        root.left = right;

        root.right = left;

        return root;

    }

}