Pop 2 view controllers in Nav Controller in Swift

Question

I have found many ways to pop back 2 UIViewControllers in UINavigationController using Objective-C, however when I try and switch that over to Swift it

Answer 1

 let allViewControllers : [UIViewController] = self.navigationController!.viewControllers as [UIViewController]
        self.navigationController?.popToViewController(allViewControllers[allViewControllers.count-3], animated: true)

Answer 2

Here is another, slightly "fool-proof" version:

extension UINavigationController {
    func popBack(_ count: Int) {
        guard count > 0 else {
            return assertionFailure("Count can not be a negative value.")
        }
        let index = viewControllers.count - count - 1
        guard index > 0 else {
            return assertionFailure("Not enough View Controllers on the navigation stack.")
        }
        popToViewController(viewControllers[index], animated: true)
    }
}