Java-队列实现

队列有多种实现的方式，有数组队列还有循环队列

• 数组队列出列的时间复杂度是O(n)

数组队列

public interface Queue<E> {
    int getSize();
    int getCapacity();
    boolean isEmpty();
    // 队列增加一个值
    void enqueue(E e);
    // 队列出列
    E dequeue();
    E getFront();
}

package dataStructure;

public class ArrayQueue<E> implements Queue<E> {
    private Array<E> array;

    public ArrayQueue(int capacity){
        array = new Array<>(capacity);
    }

    public ArrayQueue(){
        array = new Array<>();
    }

    @Override
    public int getSize() {
        return array.getSize();
    }

    @Override
    public int getCapacity() {
        return array.getCapacity();
    }

    @Override
    public boolean isEmpty() {
        return array.isEmpty();
    }

    @Override
    public void enqueue(E e) {
        array.addLast(e);
    }

    @Override
    public E dequeue() {
        return array.removeFirst();
    }

    @Override
    public E getFront() {
        return array.getFirst();
    }

    @Override
    public String toString() {
        StringBuilder res = new StringBuilder();
        res.append("ArrayQueue:");
        res.append("front [");
        for (int i = 0; i < array.getSize() ; i++) {
            res.append(array.get(i));
            if (i != array.getSize() -1){
                res.append(", ");
            }
        }
        res.append("] tail");
        return res.toString();
    }

}

循环队列

• 接口也是继承的Queue接口
• 若创建一个容量为10的队列，需要创建容量为11的数组，
  • 因为这是为了保证 front == tail时，队列为空
  • front == tail + 1时，队列为满

package dataStructure;

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

    private E data[];
    private int front;
    private int tail;
    private int size;

    public LoopQueue(int capacity) {
        data = (E[]) new Object[capacity + 1];
        front = 0;
        tail = 0;
        size = 0;
    }

    public LoopQueue() {
        this(10);
    }

    @Override
    public int getCapacity() {
        return data.length - 1;
    }

    @Override
    public boolean isEmpty() {
        return front == tail;
    }

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

    @Override
    public void enqueue(E e) {
        if ((tail + 1) % data.length == front) {
            resize(getCapacity() * 2);
        }
        data[tail] = e;
        tail = (tail + 1) % data.length;
        size++;
    }

    @Override
    public E dequeue() {
        if (isEmpty())
            throw new IllegalArgumentException("cannot dequeue from an empty LoopQueue");
        E ret = data[front];
        data[front] = null;
        front = (front + 1) % data.length;
        size--;
        if (size == getCapacity() / 4 && getCapacity() != 0) {
            resize(getCapacity() / 2);
        }
        return ret;
    }

    @Override
    public E getFront() {
        if (isEmpty())
            throw new IllegalArgumentException("cannot getFront from an empty LoopQueue");
        return data[front];
    }

    private void resize(int newCapacity) {
        E[] newData = (E[]) new Object[newCapacity + 1];
        for (int i = 0; i < size; i++) {
            newData[i] = data[(i + front) % data.length];
        }
        data = newData;
        front = 0;
        tail = size;
    }

    @Override
    public String toString() {
        StringBuilder res = new StringBuilder();
        res.append(String.format("Queue: size = %d ,capacity = %d\n", size, getCapacity()));
        res.append("front [");
        for (int i = front; i != tail; i = (i + 1) % data.length) {
            res.append(data[i]);
            if ((i + 1) % data.length != tail) {
                res.append(", ");
            }
        }
        res.append("] tail");
        return res.toString();
    }

    public static void main(String[] args) {
        LoopQueue<Integer> queue = new LoopQueue<>();
        queue.enqueue(10);
        System.out.println(queue);
    }
}

两种队列的比较

• 下面是两种队列时间复杂度的对比

链表队列

• 我们需要在链表中加个尾指针，这样的话 操作的时间复杂度会降低
• 尾端的 删除元素 时间复杂度是O(n)，添加元素 时间复杂度是O(1)
• 首端 删除、添加 的时间复杂度都是O(1)
• 所以我们选择链表的尾端添加元素，首端删除元素

package dataStructure;

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

    public 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);
        }

        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 int getCapacity() {
        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("Cannot getFront from an empty queue");
        return head.e;
    }


    @Override
    public String toString() {
        StringBuilder res = new StringBuilder();
        res.append("Queue : front ");
        Node curr = head;
        while (curr != null) {
            res.append(curr + "->");
            curr = curr.next;
        }
        res.append("Null tail");
        return res.toString();
    }

    public static void main(String[] args) {
        LinkedListQueue<Integer> queue = new LinkedListQueue();
        for (int i = 0; i < 10; i++) {
            queue.enqueue(i);
        }
        System.out.println(queue);
    }
}