使用链表实现队列

使用链表实现队列

---

public class LinkedListQueue2<E> implements Queue<E> {

    private class Node {
        public E e;
        public Node next;
        
        public Node(E e, Node next) {
            this.e = e;
            this.next = next;
        }
        
        public Node(E e) {
            this(e, null);
        }
        
        public Node() {
            this(null, null);
        }
        
        @Override
        public String toString() {
            return e.toString();
        }
    }

    private Node dummyHead, tail;
    private int size;
    
    public LinkedListQueue2() {
        dummyHead = new Node();
        tail = dummyHead;
        size = 0;
    }

    @Override
    public int getSize() {
        return size;
    }

    @Override
    public boolean isEmpty() {
        return size == 0;
    }

    @Override
    public void enqueue(E e) {

        tail.next = new Node(e);
        tail = tail.next;
    
        size++;
    }

    @Override
    public E dequeue() {
        if(this.isEmpty()) {
            throw new IllegalArgumentException("Dequeue failed. Queue is empty.");
        }
        
        Node retNode = dummyHead.next;
        dummyHead.next = retNode.next;
        retNode.next = null;
        if(dummyHead.next == null) {
            tail = dummyHead;
        }
        size--;
        return retNode.e;
    }

    @Override
    public E getFront() {
        if(isEmpty()) {
            throw new IllegalArgumentException("Queue is empty.");
        }
        return dummyHead.next.e;
    }

    @Override
    public String toString() {
        StringBuilder res = new StringBuilder();
        res.append("Queue: front ");
        Node cur = dummyHead.next;
        while(cur != null) {
            res.append(cur + "->");
            cur = cur.next;
        }
        res.append("NULL tail");
        
        return res.toString();        
    }
    
    public static void main(String[] args) {
        LinkedListQueue2<Integer> queue = new LinkedListQueue2<>();
        for(int i=0; i<10; i++) {
            queue.enqueue(i);
            System.out.println(queue);
        
            if(i % 3 == 2) {
                queue.dequeue();
                System.out.println(queue);
            }
        }
        System.out.println(queue.getSize());
        for(int i=0; i<7; i++) {
            queue.dequeue();
            System.out.println(queue);
        }
    }
}