隊列 鏈表實現 Java__Java

/** * Created by yywang on 2017/1/16. */public class Queue <Item>{    private Node first;//隊首    private Node last;  //隊尾    private int N;//元素數量    private class Node{        Item item;        Node next;    }    public boolean isEmpty() {return  first==null;}    public int size(){return N;}    public void enqueue(Item item) {        Node oldlast = last;        Node last = new Node();        last.item = item;        last.next = null;        if (isEmpty()){            first = last;        }else{            oldlast.next=last;        }        ;        N++;       }    public Item dequeue(){        Item item = first.item;        first = first.next;        if(isEmpty()){            last = null;        }        N--;        return item;    }}