玩转数据结构之链表应用：实现队列

队列是队首删除元素，队尾添加元素，当我们用链表来实现队列时，如果从链表尾删除元素是需要先遍历到尾巴之前的一个元素，即时间复杂度不是O(1)是O(n)，因此对链表来说，从链表头删除元素是很好操作的，故将链表头当作队首。

带尾指针的链表具体实现：

public class LinkedListQueue <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 head, tail;
    private int size;

    public LinkedListQueue(){
        head = null;
        tail = null;
        size = 0;
    }

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

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

    @Override
    public void enqueue(E e){
        if (tail == null){
            tail = new Node(e);
            head =tail;
        }
        else{
            tail.next = new Node(e);
            tail = tail.next;
        }
        size ++;
    }

    @Override
    public E dequeue(){
        if (isEmpty())
            throw new IllegalArgumentException("cannot dequeue from an empty queue");

        Node retNode = head;
        head =head.next;
        retNode.next = null;
        if (head == null)
            tail = null;
        size --;
        return retNode.e;
    }

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

    @Override
    public String toString(){
        StringBuilder res = new StringBuilder();
        res.append("Queue: front");

        Node cur =head;
        while(cur != null){
            res.append(cur + "->");
            cur = cur.next;
        }
        res.append("NULL tail");
        return res.toString();
    }
}

使用动态数组实现的队列，时间复杂度为O(n)，耗时最长

循环数组实现的队列和链表实现的队列时间复杂度相同