队列

队列是一种先进先出的数据结构。

public interface Queue<E> {

    /**
     * 入队
     *
     * @param e 元素
     */
    void enqueue(E e);

    /**
     * 出队
     *
     * @return 队首元素
     */
    E dequeue();

    /**
     * 查看队首元素
     *
     * @return 队首元素
     */
    E getFront();

    /**
     * 队列里是否包含元素
     *
     * @return 是否包含
     */
    boolean isEmpty();

    /**
     * 获取队列里元素的个数
     *
     * @return 队列元素个数
     */
    int getSize();

}

基于动态数组的队列

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

    /**
     * 底层基于动态数组
     */
    private Array<E> data;

    /**
     * 构造函数，指定容量
     *
     * @param capacity 容量
     */
    public ArrayQueue(int capacity) {
        data = new Array<>(capacity);
    }

    /**
     * 无参构造函数，默认容量为10
     */
    public ArrayQueue() {
        this(10);
    }

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

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

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

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

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

    /**
     * 获取容量
     *
     * @return 容量
     */
    public int getCapacity() {
        return data.getCapacity();
    }

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

}

循环队列

/**
 * 循环队列 front == tail 时队列为空；tail + 1 == front时队列满
 */
public class LoopQueue<E> implements Queue<E> {

    /**
     * 底层基于静态数组
     */
    private E[] data;
    /**
     * 队首索引
     */
    private int front;
    /**
     * 队尾索引
     */
    private int tail;
    /**
     * 元素个数
     */
    private int size;

    /**
     * 构造函数，指定容量
     *
     * @param capacity 容量
     */
    public LoopQueue(int capacity) {
        data = (E[]) new Object[capacity + 1];
        front = 0;
        tail = 0;
        size = 0;
    }

    /**
     * 无参构造函数，默认容量为10
     */
    public LoopQueue() {
        this(10);
    }

    @Override
    public void enqueue(E e) {
        if ((tail + 1) % data.length == front) {
            //循环队列满了，扩容
            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 queue.");
        }

        E ret = data[front];
        data[front] = null;

        front = (front + 1) % data.length;
        size--;

        if (size == getCapacity() / 4 && getCapacity() / 2 != 0) {
            resize(getCapacity() / 2);
        }

        return ret;
    }

    @Override
    public E getFront() {
        if (isEmpty()) {
            throw new IllegalArgumentException("Queue is Empty.");
        }
        return data[front];
    }

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

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

    /**
     * 获取容量
     *
     * @return 容量
     */
    public int getCapacity() {
        return data.length - 1;
    }

    /**
     * 扩容或缩容
     *
     * @param newCapacity 新的容量
     */
    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();
    }

}

基于链表的队列

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();
        }
    }

    /**
     * 队首
     */
    private Node head;
    /**
     * 队尾
     */
    private Node tail;
    /**
     * 元素个数
     */
    private int size;

    /**
     * 无参构造器
     */
    public LinkedListQueue() {
        head = null;
        tail = null;
        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("Queue is empty.");
        }

        Node node = head;
        head = head.next;
        node.next = null;
        size--;

        if (head == null) {
            tail = null;
        }

        return node.e;
    }

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

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

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

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

}