导航栏隐藏，下一级返回显示异常的问题

代码如下：

- (void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];
    [self.navigationController setNavigationBarHidden:YES animated:YES];
}

- (void)viewWillDisappear:(BOOL)animated {
    self.navigationController.navigationBar.hidden = NO;
    [super viewWillDisappear:animated];
}