摘要：我理解的數(shù)據(jù)結(jié)構(gòu)三隊(duì)列一隊(duì)列隊(duì)列是一種線性結(jié)構(gòu)相比數(shù)組，隊(duì)列對(duì)應(yīng)的操作是數(shù)組的子集只能從一端隊(duì)尾添加元素，只能從另一端隊(duì)首取出元素隊(duì)列是一種先進(jìn)先出的數(shù)據(jù)結(jié)構(gòu)二數(shù)組隊(duì)列與循環(huán)隊(duì)列數(shù)組隊(duì)列如果你有看過我之前的文章不要小看了數(shù)組或者棧，你就會(huì)發(fā)

隊(duì)列是一種線性結(jié)構(gòu)

相比數(shù)組，隊(duì)列對(duì)應(yīng)的操作是數(shù)組的子集

只能從一端（隊(duì)尾）添加元素，只能從另一端（隊(duì)首）取出元素

隊(duì)列是一種先進(jìn)先出的數(shù)據(jù)結(jié)構(gòu)（FIFO）

如果你有看過我之前的文章不要小看了數(shù)組或者棧，你就會(huì)發(fā)現(xiàn)，自己封裝一個(gè)數(shù)組隊(duì)列是如此的輕松加愉快！

（1）先定義一個(gè)接口，接口中定義隊(duì)列需要實(shí)現(xiàn)的方法

public interface Queue {
    int getSize();
    boolean isEmpty();
    // 查看隊(duì)首元素
    E getFront();
    // 入隊(duì)
    void enqueue(E ele);
    // 出隊(duì)
    E dequeue();
}

（2）實(shí)現(xiàn)數(shù)組隊(duì)列

public class ArrayQueue implements Queue {
    
    // 這里的數(shù)組是在之前的文章中封裝好的，直接拿來用就好了
    private ArrayNew array;

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

    public ArrayQueue() {
        this(10);
    }

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

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

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

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

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

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

    @Override
    public String toString() {

        StringBuffer res = new StringBuffer();

        res.append(String.format("arrayQueue: size = %d, capacity = %d
", getSize(), getCapacity()));
        res.append("front [");

        for (int i = 0; i < array.getSize(); i++) {
            res.append(array.get(i));
            if (i != getSize() - 1) {
                res.append(", ");
            }
        }
        res.append("] tail");
        return res.toString();

    }

}

（3）數(shù)組隊(duì)列的復(fù)雜度

這個(gè)時(shí)候我們會(huì)發(fā)現(xiàn)，在進(jìn)行出隊(duì)操作的時(shí)候，數(shù)組隊(duì)列的復(fù)雜度是0(n)，如果我們頻繁的進(jìn)行出隊(duì)操作，那么其實(shí)數(shù)組隊(duì)列的效率是很低的，如何提升數(shù)組隊(duì)列的性能呢？這個(gè)時(shí)候我們就要用到循環(huán)隊(duì)列了。

循環(huán)隊(duì)列的原理：

dequeue時(shí)，不要在去除隊(duì)首元素時(shí)，把整體向前移動(dòng)

維護(hù) front 、 tail 和 size 這三個(gè)屬性

enqueue的時(shí)候tail++

dequeue的時(shí)候front++

（1）實(shí)現(xiàn)循環(huán)隊(duì)列

public class LoopQueue implements Queue {

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

    public LoopQueue(int capacity) {
        // 我們需要浪費(fèi)一個(gè)空間去判斷隊(duì)列是否已滿，所以需要把capacity + 1
        array = (E[])new Object[capacity + 1];
        front = 0;
        tail = 0;
        size = 0;
    }

    public LoopQueue() {
        this(10);
    }

    // 返回用戶傳遞的隊(duì)列大小
    public int getCapacity() {
        return array.length - 1;
    }

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

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

    @Override
    public E getFront() {
        if (isEmpty()) {
            throw new IllegalArgumentException("Queue is empty. Can"t get front.");
        }

        return array[0];
    }

    @Override
    public void enqueue(E ele) {

        if (front == (tail + 1) % array.length) {
            // 擴(kuò)展隊(duì)列長(zhǎng)度為原長(zhǎng)度2倍
            resize(getCapacity() * 2);
        }

        array[tail] = ele;
        size++;
        tail = (tail + 1) % array.length;
    }

    @Override
    public E dequeue() {

        if (isEmpty()) { // 隊(duì)列為空
            throw new IllegalArgumentException("Queue is empty. Can"t get dequeue.");
        }

        E ele = array[front];

        size--;
        array[front] = null;
        front = (front + 1) % array.length;

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

        return ele;

    }

    private void resize(int newCapacity) {
        E[] newArray = (E[]) new Object[newCapacity + 1];

        for (int i = 0; i < size; i++) {
            newArray[i] = array[(front + i) % array.length];
        }

        array = newArray;
        front = 0;
        tail = size;
    }

    @Override
    public String toString() {
        StringBuffer res = new StringBuffer();

        res.append(String.format("queue: size = %d, capacity = %d
", getSize(), getCapacity()));
        res.append("front [");

        // 循環(huán)條件，和循環(huán)增量都要注意下
        for (int i = front; i != tail; i = (i + 1) % array.length) {
            res.append(array[i]);

            if ((i + 1) % array.length != tail) {
                res.append(", ");
            }
        }
        res.append("] tail");

        return res.toString();
    }

}

（2）循環(huán)隊(duì)列的復(fù)雜度

（1）用時(shí)方法

public static double test(Queue q, int opCount) {

    // 納秒
    long startTime = System.nanoTime();

    Random random = new Random();

    for (int i = 0; i < opCount; i++) {
        q.enqueue(random.nextInt(Integer.MAX_VALUE));
    }
    for (int i = 0; i < opCount; i++) {
        q.dequeue();
    }

    // 納秒
    long endTime = System.nanoTime();

    return (endTime - startTime) / 1000000000.0;
}

（2）調(diào)用

// 十萬次入隊(duì)和十萬次出隊(duì)操作
int opCount = 100000;

ArrayQueue aq = new ArrayQueue<>();
double time1 = test(aq, opCount);
System.out.println(time1);

LoopQueue lq = new LoopQueue<>();
double time2 = test(lq, opCount);
System.out.println(time2);

（3）結(jié)果

14.635995113

0.054536447

這個(gè)就是算法和數(shù)據(jù)結(jié)構(gòu)的力量！