如果前一个视图没有标题，如何使导航栏后退按钮总是说后退？

override func viewDidLoad() {
    super.viewDidLoad()
    
    let backButton = UIBarButtonItem()
    backButton.title = "后退"
    self.navigationItem.backBarButtonItem = backButton
}

override func viewDidLoad() {
    super.viewDidLoad()
    
    let backButton = UIBarButtonItem(title: "后退", style: .plain, target: nil, action: nil)
    self.navigationItem.leftBarButtonItem = backButton
}