如何在不使用Storyboard的情况下以编程方式导航Objective-C App？

UIViewController *rootViewController = [[UIViewController alloc] init];
UINavigationController *navigationController = [[UINavigationController alloc] initWithRootViewController:rootViewController];

UIViewController *secondViewController = [[UIViewController alloc] init];
[self.navigationController pushViewController:secondViewController animated:YES];

[self.navigationController popViewControllerAnimated:YES];

UIViewController *modalViewController = [[UIViewController alloc] init];
[self presentViewController:modalViewController animated:YES completion:nil];