class Stack {
    var stack: [AnyObject]
    var isEmpty: Bool { return stack.isEmpty }
    var peek: AnyObject? { return stack.last }
    
    int() {
      stack = [AnyObject]()
    }
    
    func push(object: AnyObject) {
        stack.append(object)
    }

    func pop() -> AnyObject? {
        if (!isEmpty) {
            return stack.removeLast()
        }else {
            return nil
        }
    }
}