title: UINavigationController, how to hide tabbar in second level viewController then show tabbar in third level viewController
body: <p>here is a piece of my code, but in this way, when I push the third level view controller, the tabbar won't show.  </p>
  
  <pre><code>//at first level
  SecondLevelViewController *_2vc = [[SecondLevelViewController alloc]initWithNibName:@"SecondLevelViewController" bundle:nil];
      _2vc.hidesBottomBarWhenPushed = YES;
      [self.navigationController pushViewController:_2vc animated:YES];  
  
  //at second level
  ThirdLevelViewController *_3vc = [[ThirdLevelViewController alloc]initWithNibName:@"ThirdLevelViewController" bundle:nil];
      _3vc.hidesBottomBarWhenPushed = NO;
      [self.navigationController pushViewController:_3vc animated:YES];
  </code></pre>