UINavigationController 返回任意目录
返回上一级
[self.navigationController popViewControllerAnimated:YES];
返回第一级
popToRootViewControllerAnimated:YES
返回到中间任意一级
[self.navigationController popToViewController: [self.navigationController.viewControllers objectAtIndex: ([self.navigationController.viewControllers count] -2)] animated:YES];