方案1.先取消modal出來的窗口，取得對應的導航控制器，讓后push新的控制器

  // 先取消原來的控制器
  [self dismissViewControllerAnimated:NO completion:nil];
  ViewController *VC1 = [[ViewController alloc] init];
  UIWindow *kwindow = [UIApplication sharedApplication].keyWindow;
  // 取得窗口的導航控制器
  UINavigationController *nav = (UINavigationController *)kwindow.rootViewController;
  NSLog(@"%@",nav.childViewControllers);
  MyNavigationController *lastNavc = [nav.viewControllers lastObject];
  // 導航控制器
  [lastNavc pushViewController: VC1 animated:YES];

方案2.modal窗口的基礎上，繼續modal方式出來一個導航控制器

  ViewController * vc1 = [[ViewController alloc] init];
  UINavigationController *nav = [[UINavigationController alloc] initWithRootViewController: vc1];
  [self presentViewController:nav animated:YES completion:nil];

  // 壓入導航控制器棧的控制器，可以按照導航控制器的規則依次入棧和出棧。
  在vc1頁面push出vc2
  ViewController * vc2 = [[ViewController alloc] init];
  [self.navigationController pushViewController: vc2 animated:YES];

  // 在導航控制器棧的任何一個頁面都可以modal的方式直接返回
  [self dismissViewControllerAnimated:YES completion:nil];

方案3.push的基礎上modal

  ViewController * vc2 = [[ViewController alloc] init];
  [self.navigationController pushViewController: vc2 animated:YES];

  ViewController * vc1 = [[ViewController alloc] init];
  [self presentViewController:vc1 animated:YES completion:nil];