iOS Swift. How Programmatically go back to previous ViewController

To go back to Root View Controller:

@IBAction func goToFirst(_ sender: AnyObject) {
        let _ = self.navigationController?.popToRootViewController(animated: true)
    }

To go back to previous View Controller:

@IBAction func showPrev(_ sender: AnyObject) {
        let _ = self.navigationController?.popViewController(animated: true)
    }