How to implement NSUndoManager and arrays in iOS using Swift 2 - ios

Question

So quick question. I am writing an iOS app for a dart game. And I am trying to implement an undo button to undo the last action.
Whats going on: When you push any of the buttons they add that amount to the score. The users score that the buttons have an effect on is the currentPlayer.
I want the undo button to undo whatever button was pushed last. I have been researching and it seems like they are all showing how to hard code an undo action. However it could be any of the 6 buttons hit_single, hit_double, attack_double... etc.
FIRST: I don't know how to use undoManager.registerUndoWithTarget(target:AnyObject, selector: Selector, object: AnyObject?), I'm not sure how to apply this and what each of these 3 things do.
SECOND: In my game you have to go from 1-20(the numbers on a dart board) and then hit bullseye to win. The variables user1score and user2score increase their count to display what number the player is aiming at. However, after 20 I want the score to display "Bullseye". I am not sure if there is a way to write a mixed content array, one that allows counting 1-20, and then the 20th array value say "Bullseye". Or maybe write an If statement that says if the user1score/user2score is greater than 20 print "Bullseye". The problem is it says its an integer and I cant input a string value.
Here is my code:
import UIKit
class Galaxy_VC: UIViewController {
#IBOutlet var currentPlayerName: UILabel!
#IBOutlet weak var user1score: UILabel!
#IBOutlet weak var user2score: UILabel!
#IBOutlet weak var darts_Thrown: UILabel!
var names = ["Big Meat", "J Hooks"]
var currentPlayer = 0
var scores = [0, 0]
var count = 0
override func viewDidLoad() {
super.viewDidLoad()
setupGame()
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
// Dispose of any resources that can be recreated.
}
func setupGame() {
user1score.text = "\(count)"
user2score.text = "\(count)"
}
func updateTurn() {
currentPlayerName.text = names[currentPlayer]
user1score.text = "\(scores[0])"
user2score.text = "\(scores[1])"
}
func undoLast(score: NSNumber) {
undoManager?.registerUndoWithTarget(self, handler: <#T##AnyObject -> ()#>)
}
#IBAction func btn_NextTurn(sender: UIButton) {
currentPlayer = 1 - currentPlayer
updateTurn()
}
#IBAction func btn_UndoTurn(sender: AnyObject) {
}
#IBAction func hit_Single(sender: AnyObject) {
scores[currentPlayer] + 1
updateTurn()
}
#IBAction func hit_Double(sender: AnyObject) {
scores[currentPlayer] + 2
updateTurn()
}
#IBAction func hit_Triple(sender: AnyObject) {
scores[currentPlayer] + 3
updateTurn()
}
#IBAction func Miss(sender: AnyObject) {
scores[currentPlayer] - 1
updateTurn()
}
#IBAction func attack_double(sender: AnyObject) {
scores[1 - currentPlayer] - 2
updateTurn()
}
#IBAction func attack_Triple(sender: AnyObject) {
scores[1 - currentPlayer] - 3
updateTurn()
}
}
Any help you can provide would be really appreciated! Thanks!