public void Mirror(TreeNode root) {

    if (root == null) {

        return ;

    }

    if (root.left == null && root.right == null) {

        return;

    }

    TreeNode temp = root.left;

    root.left = root.right;

    root.right = temp;

    if(root.left!=null){

        Mirror(root.left);

    }

    if(root.right!=null){

        Mirror(root.right);

    }

}