iOS presentViewController两个视图控制器的时候返回根视图

 if ([self respondsToSelector:@selector(presentingViewController)]){        self.presentingViewController.view.alpha = 0;        [self.presentingViewController.presentingViewController dismissViewControllerAnimated:YES completion:nil];    }else {        self.presentingViewController.view.alpha = 0;        [self.parentViewController.parentViewController dismissViewControllerAnimated:YES completion:nil];    }