1.导航（可逆）
[self.navigationController pushViewController:视图控制器 animated:YES];
[self.navigationController popViewControllerAnimated:YES];
2.模态（可逆）
[self presentViewController:视图控制器 animated:YES completion:nil];
[self dismissViewControllerAnimated:YES completion:nil];
3.切换根视图控制器（不可逆）
UIApplication sharedApplication].keyWindow.rootViewController = 视图控制器;
