解決alertController無法像alertView那樣重覆推出來的問題

UIViewController *presentingViewController = [[[UIApplication sharedApplication] delegate] window].rootViewController;

        //因為一個ViewController只能present新的VC一次，故如果它present過了，就去抓取它present出來的VC來再present新的VC

        if (presentingViewController.presentedViewController != nil)

        {

            presentingViewController = presentingViewController.presentedViewController;

        }

        [presentingViewController presentViewController:alertController animated:YES completion:nil];