队列Queue的实现

数组实现

package DataStructures.Queues;

/**
 * This implements Queues by using the class Queue.
 * <p>
 * A queue data structure functions the same as a real world queue.
 * The elements that are added first are the first to be removed.
 * New elements are added to the back/rear of the queue.
 *
 */
class Queue {
    /**
     * Default initial capacity.
     */
    private static final int DEFAULT_CAPACITY = 10;

    /**
     * Max size of x`the queue
     */
    private int maxSize;
    /**
     * The array representing the queue
     */
    private int[] queueArray;
    /**
     * Front of the queue
     */
    private int front;
    /**
     * Rear of the queue
     */
    private int rear;
    /**
     * How many items are in the queue
     */
    private int nItems;

    /**
     * init with DEFAULT_CAPACITY
     */
    public Queue() {
        this(DEFAULT_CAPACITY);
    }

    /**
     * Constructor
     *
     * @param size Size of the new queue
     */
    public Queue(int size) {
        maxSize = size;
        queueArray = new int[size];
        front = 0;
        rear = -1;
        nItems = 0;
    }

    /**
     * Inserts an element at the rear of the queue
     *
     * @param x element to be added
     * @return True if the element was added successfully
     */
    public boolean insert(int x) {
        if (isFull())
            return false;
        // If the back of the queue is the end of the array wrap around to the front
        rear = (rear + 1) % maxSize;
        queueArray[rear] = x;
        nItems++;
        return true;
    }

    /**
     * Remove an element from the front of the queue
     *
     * @return the new front of the queue
     */
    public int remove() {
        if (isEmpty()) {
            return -1;
        }
        int temp = queueArray[front];
        front = (front + 1) % maxSize;
        nItems--;
        return temp;
    }

    /**
     * Checks what's at the front of the queue
     *
     * @return element at the front of the queue
     */
    public int peekFront() {
        return queueArray[front];
    }

    /**
     * Checks what's at the rear of the queue
     *
     * @return element at the rear of the queue
     */
    public int peekRear() {
        return queueArray[rear];
    }

    /**
     * Returns true if the queue is empty
     *
     * @return true if the queue is empty
     */
    public boolean isEmpty() {
        return nItems == 0;
    }

    /**
     * Returns true if the queue is full
     *
     * @return true if the queue is full
     */
    public boolean isFull() {
        return nItems == maxSize;
    }

    /**
     * Returns the number of elements in the queue
     *
     * @return number of elements in the queue
     */
    public int getSize() {
        return nItems;
    }

    @Override
    public String toString() {
        StringBuilder sb = new StringBuilder();
        sb.append("[");
        for (int i = front; ; i = ++i % maxSize) {
            sb.append(queueArray[i]).append(", ");
            if (i == rear) {
                break;
            }
        }
        sb.replace(sb.length() - 2, sb.length(), "]");
        return sb.toString();
    }
}

/**
 * This class is the example for the Queue class
 *
 * @author Unknown
 */
public class Queues {
    /**
     * Main method
     *
     * @param args Command line arguments
     */
    public static void main(String[] args) {
        Queue myQueue = new Queue(4);
        myQueue.insert(10);
        myQueue.insert(2);
        myQueue.insert(5);
        myQueue.insert(3);
        // [10(front), 2, 5, 3(rear)]

        System.out.println(myQueue.isFull()); // Will print true

        myQueue.remove(); // Will make 2 the new front, making 10 no longer part of the queue
        // [10, 2(front), 5, 3(rear)]

        myQueue.insert(7); // Insert 7 at the rear which will be index 0 because of wrap around
        // [7(rear), 2(front), 5, 3]

        System.out.println(myQueue.peekFront()); // Will print 2
        System.out.println(myQueue.peekRear()); // Will print 7
        System.out.println(myQueue.toString()); // Will print [2, 5, 3, 7]
    }
}

链表实现

package DataStructures;

import java.util.NoSuchElementException;

public class LinkedQueue {
    class Node {
        int data;
        Node next;

        public Node() {
            this(0);
        }

        public Node(int data) {
            this(data, null);
        }

        public Node(int data, Node next) {
            this.data = data;
            this.next = next;
        }
    }

    /**
     * Front of Queue
     */
    private Node front;

    /**
     * Rear of Queue
     */
    private Node rear;

    /**
     * Size of Queue
     */
    private int size;

    /**
     * Init LinkedQueue
     */
    public LinkedQueue() {
        front = rear = new Node();
    }

    /**
     * Check if queue is empty
     *
     * @return <tt>true</tt> if queue is empty, otherwise <tt>false</tt>
     */
    public boolean isEmpty() {
        return size == 0;
    }

    /**
     * Add element to rear of queue
     *
     * @param data insert value
     * @return <tt>true</tt> if add successfully
     */
    public boolean enqueue(int data) {
        Node newNode = new Node(data);
        rear.next = newNode;
        rear = newNode; /* make rear point at last node */
        size++;
        return true;
    }

    /**
     * Remove element at the front of queue
     *
     * @return element at the front of queue
     */
    public int dequeue() {
        if (isEmpty()) {
            throw new NoSuchElementException("queue is empty");
        }
        Node destroy = front.next;
        int retValue = destroy.data;
        front.next = front.next.next;
        destroy = null; /* clear let GC do it's work */
        size--;

        if (isEmpty()) {
            front = rear;
        }

        return retValue;
    }

    /**
     * Peek element at the front of queue without removing
     *
     * @return element at the front
     */
    public int peekFront() {
        if (isEmpty()) {
            throw new NoSuchElementException("queue is empty");
        }
        return front.next.data;
    }

    /**
     * Peek element at the rear of queue without removing
     *
     * @return element at the front
     */
    public int peekRear() {
        if (isEmpty()) {
            throw new NoSuchElementException("queue is empty");
        }
        return rear.data;
    }

    /**
     * Return size of queue
     *
     * @return size of queue
     */
    public int size() {
        return size;
    }

    /**
     * Clear all nodes in queue
     */
    public void clear() {
        while (!isEmpty()) {
            dequeue();
        }
    }

    @Override
    public String toString() {
        if (isEmpty()) {
            return "[]";
        }
        StringBuilder builder = new StringBuilder();
        Node cur = front.next;
        builder.append("[");
        while (cur != null) {
            builder.append(cur.data).append(", ");
            cur = cur.next;
        }
        builder.replace(builder.length() - 2, builder.length(), "]");
        return builder.toString();
    }

    /* Driver Code */
    public static void main(String[] args) {
        LinkedQueue queue = new LinkedQueue();
        assert queue.isEmpty();

        queue.enqueue(1); /* 1 */
        queue.enqueue(2); /* 1 2 */
        queue.enqueue(3); /* 1 2 3 */
        System.out.println(queue); /* [1, 2, 3] */

        assert queue.size() == 3;
        assert queue.dequeue() == 1;
        assert queue.peekFront() == 2;
        assert queue.peekRear() == 3;

        queue.clear();
        assert queue.isEmpty();

        System.out.println(queue); /* [] */
    }
}

优先级队列，这里采用的是整体移动，好像数组也只能这么做

package DataStructures.Queues;

/**
 * This class implements a PriorityQueue.
 * <p>
 * A priority queue adds elements into positions based on their priority.
 * So the most important elements are placed at the front/on the top.
 * In this example I give numbers that are bigger, a higher priority.
 * Queues in theory have no fixed size but when using an array
 * implementation it does.
 */
class PriorityQueue {
    /**
     * The max size of the queue
     */
    private int maxSize;
    /**
     * The array for the queue
     */
    private int[] queueArray;
    /**
     * How many items are in the queue
     */
    private int nItems;

    /**
     * Constructor
     *
     * @param size Size of the queue
     */
    public PriorityQueue(int size) {
        maxSize = size;
        queueArray = new int[size];
        nItems = 0;
    }

    /**
     * Inserts an element in it's appropriate place
     *
     * @param value Value to be inserted
     */
    public void insert(int value) {
        if (isFull()) {
            throw new RuntimeException("Queue is full");
        } else {
            int j = nItems - 1; // index of last element
            while (j >= 0 && queueArray[j] > value) {
                queueArray[j + 1] = queueArray[j]; // Shifts every element up to make room for insertion
                j--;
            }
            queueArray[j + 1] = value; // Once the correct position is found the value is inserted
            nItems++;
        } 
    }

    /**
     * Remove the element  from the front of the queue
     *
     * @return The element removed
     */
    public int remove() {
        return queueArray[--nItems];
    }

    /**
     * Checks what's at the front of the queue
     *
     * @return element at the front of the queue
     */
    public int peek() {
        return queueArray[nItems - 1];
    }

    /**
     * Returns true if the queue is empty
     *
     * @return true if the queue is empty
     */
    public boolean isEmpty() {
        return (nItems == 0);
    }

    /**
     * Returns true if the queue is full
     *
     * @return true if the queue is full
     */
    public boolean isFull() {
        return (nItems == maxSize);
    }

    /**
     * Returns the number of elements in the queue
     *
     * @return number of elements in the queue
     */
    public int getSize() {
        return nItems;
    }
}

/**
 * This class implements the PriorityQueue class above.
 *
 * @author Unknown
 */
public class PriorityQueues {
    /**
     * Main method
     *
     * @param args Command Line Arguments
     */
    public static void main(String[] args) {
        PriorityQueue myQueue = new PriorityQueue(4);
        myQueue.insert(10);
        myQueue.insert(2);
        myQueue.insert(5);
        myQueue.insert(3);
        // [2, 3, 5, 10] Here higher numbers have higher priority, so they are on the top

        for (int i = 3; i >= 0; i--)
            System.out.print(myQueue.remove() + " "); // will print the queue in reverse order [10, 5, 3, 2]

        // As you can see, a Priority Queue can be used as a sorting algotithm
    }
}

ArrayList实现，add加最后，get随意，remove随意，很简单了

package DataStructures.Queues;

import java.util.ArrayList;

/**
 * This class implements a GenericArrayListQueue.
 * <p>
 * A GenericArrayListQueue data structure functions the same as any specific-typed queue.
 * The GenericArrayListQueue holds elemets of types to-be-specified at runtime.
 * The elements that are added first are the first to be removed (FIFO)
 * New elements are added to the back/rear of the queue.
 *
 */
public class GenericArrayListQueue<T> {
    /**
     * The generic ArrayList for the queue
     * T is the generic element
     */
    ArrayList<T> _queue = new ArrayList<T>();

    /**
     * Checks if the queue has elements (not empty)
     *
     * @return True if the queue has elements. False otherwise.
     */
    private boolean hasElements() {
        return !_queue.isEmpty();
    }

    /**
     * Checks what's at the front of the queue
     *
     * @return If queue is not empty, element at the front of the queue. Otherwise, null
     */
    public T peek() {
        T result = null;
        if(this.hasElements()) { result = _queue.get(0); }
        return result;
    }

    /**
     * Inserts an element of type T to the queue.
     *
     * @param element of type T to be added
     * @return True if the element was added successfully
     */
    public boolean add(T element) {
        return _queue.add(element);
    }

    /**
     * Retrieve what's at the front of the queue
     *
     * @return If queue is not empty, element retrieved. Otherwise, null
     */
    public T poll() {
        T result = null;
        if(this.hasElements()) { result = _queue.remove(0); }
        return result;
    }

    /**
     * Main method
     *
     * @param args Command line arguments
     */
    public static void main(String[] args) {
        GenericArrayListQueue<Integer> queue = new GenericArrayListQueue<Integer>();
        System.out.println("Running...");
        assert queue.peek() == null;
        assert queue.poll() == null;
        assert queue.add(1) == true;
        assert queue.peek() == 1;
        assert queue.add(2) == true;
        assert queue.peek() == 1;
        assert queue.poll() == 1;
        assert queue.peek() == 2;
        assert queue.poll() == 2;
        assert queue.peek() == null;
        assert queue.poll() == null;
        System.out.println("Finished.");
    }
}

暂时实现这些，队列其实是个常用的在业务场景。要理解。