java 数据结构 队列

队列是一种基于先进先出(FIFO)的数据结构，是一种只能在一端进行插入,在另一端进行删除操作的特殊线性表，它
按照先进先出的原则存储数据，先进入的数据，在读取数据时先读被读出来。

public class Queue implements Iterable {
    //记录首结点
    private Node head;
    //记录最后一个结点
    private Node last;
    //记录队列中元素的个数
    private int N;

    public Queue() {
        head = new Node(null, null);
        last = null;
        N = 0;
    }

    //判断队列是否为空
    public boolean isEmpty() {
        return N == 0;
    }

    //返回队列中元素的个数
    public int size() {
        return N;
    }

    //向队列中插入元素t
    public void enqueue(T t) {
        if (last == null) {
            last = new Node(t, null);
            head.next = last;
        } else {
            Node oldLast = last;
            last = new Node(t, null);
            oldLast.next = last;
        }
//个数+1
        N++;
    }

    //从队列中拿出一个元素
    public T dequeue() {
        if (isEmpty()) {
            return null;
        }
        Node oldFirst = head.next;
        head.next = oldFirst.next;
        N--;
        if (isEmpty()) {
            last = null;
        }
        return oldFirst.item;
    }

    @Override
    public Iterator iterator() {
        return new QIterator();
    }

    private class QIterator implements Iterator {
        private Node n = head;

        @Override
        public boolean hasNext() {
            return n.next != null;
        }

        @Override
        public T next() {
            Node node = n.next;
            n = n.next;
            return node.item;
        }
    }

    private class Node {
        public T item;
        public Node next;

        public Node(T item, Node next) {
            this.item = item;
            this.next = next;
        }
    }
}