override func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
// 1: try loading the "Detail" view controller and typecasting it to be DetailViewController
if let vc = storyboard?.instantiateViewController(withIdentifier: "Detail") as? DetailViewController {
// 2: success! Set its selectedImage property
vc.selectedImage = pictures[indexPath.row]
// 3: now push it onto the navigation controller
navigationController?.pushViewController(vc, animated: true)
}
}