swift创建栈

protocol Stack {
    associatedtype Element
    var isEmpty : Bool {get}
    
    var size : Int {get}
    //栈顶元素
    var peek : Element?{get}
    
     func push(_ newElement : Element)
    
     func pop() -> Element?
}

class IntegerStack : Stack {
    typealias Element = Int
    
    var isEmpty: Bool{
        return stack.isEmpty
    }
    
    var size: Element{
        return stack.count
    }
    
    var peek: Element?{
        return stack.last
    }
    
     func push(_ newElement: Element) {
        stack.append(newElement)
    }
    
     func pop() -> Element? {
        return stack.popLast()
    }
    
    var stack = [Element]()
}