场景：A-->B-->C 跳转到C时，要关掉B的处理方法

场景：A-->B-->C 跳转到C时，要关掉B的处理方法：相当于从A跳转到C

UIViewController *preController = [self.navigationController.viewControllers objectAtIndex:self.navigationController.viewControllers.count-2];//preController指向A
//关闭当前页面
[self.navigationController popViewControllerAnimated:FALSE];
//进入考试
PaperCoreController * controller = [[PaperCoreController alloc] init];
[controller setExamId:_examModel.id epaId:_examModel.epaId examName:_examModel.examName examDuration:_examModel.examDuration bestSubmit:_examModel.bestSubmit.intValue isExam:TRUE];
[preController.navigationController pushViewController:controller animated:TRUE ];