struct Stack<Element> {
    var items = [Element]()

    mutating func push(_ item: Element) {
        items.append(item)
    }

    mutating func pop() -> Element? {
        return items.popLast()
    }
}

var stackOfInts = Stack<Int>()
stackOfInts.push(1)
stackOfInts.push(2)
stackOfInts.push(3)

let poppedInt = stackOfInts.pop()