swift创建队列

protocol Queue{
    //持有元素类型
    associatedtype  Element
    //是否为空
    var isEmpty : Bool{get}
    //队首元素
    var peek : Element?{get}
    //队列长度
    var size : Int {get}
    //进队
    mutating func enqueue(_ element : Element)
    //出队
    mutating func dequeue() -> Element?
}

struct IntegerQueue : Queue{
    
    
    typealias Element = Int
    //是否为空
    var isEmpty : Bool{return left.isEmpty && right.isEmpty}
    //队首元素
    var peek : Element?{left.isEmpty ? right.first : left.last}
    //队列长度
    var size : Int {return left.count + right.count}
    //进队
    mutating func enqueue(_ element : Element){
        right.append(element)
    }
    //出队
    mutating func dequeue() -> Element?{
        if left.isEmpty{
            left = right.reversed()
            right.removeAll()
        }
        return left.popLast()
    }
    
    var left = [Element]()
    
    var right = [Element]()
    
}