iOS UIViewcontroller直接dismiss回根视

话不多说，直接上代码，只看代码就很好理解了：

- (void)backToRootViewcontroller{
  UIViewController *rootVC = self.presentingViewController;
  while (rootVC.presentingViewController) {
      rootVC = rootVC.presentingViewController;
  }
  [rootVC dismissViewControllerAnimated:YES completion:nil];
}