UINavigationController如何连续返回两级／回到第一级

//连续返回两级
int index=[[self.navigationController viewControllers]indexOfObject:self];
[self.navigationController popToViewController:[self.navigationController.viewControllers objectAtIndex:index-2]animated:YES];



//切换根
AppDelegate *app = [[UIApplication sharedApplication]delegate];
        app.window.rootViewController = [[BaseTabbarController alloc]init];