Defining initial controller without storyboard

AppDelegate.swift

var window: UIWindow?
func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {

    window = UIWindow(frame: UIScreen.mainScreen().bounds)
    window?.rootViewController = UINavigationController(rootViewController: PetsViewController())
    window?.makeKeyAndVisible()

    return true
}