循环队列

数组队列出队的时间复杂度为 O(n) 循环队列出队为 O(1)（均摊时间复杂度，因为缩容时时间复杂度还是 O(n)）
数组队列在删除队首元素时后面的元素要往前移动，比较耗时，循环队列使用 front 指针后移无需移动后面元素，
出队腾出的空间会在最后位置填充时利用。

tail == front 时队列为空，tail 指针要保证指向空间没有元素

实现

public class LoopQueue<E> implements Queue<E> {
    private E[] data;
    private int front, tail;
    private int size;
    public LoopQueue(int capacity) {
        data = (E[])new Object[capacity + 1];
        front = 0;
        tail = 0;
        size = 0;
    }
    public LoopQueue() {
        this(10);
    }
    public int getCapacity() {
        return data.length - 1;
    }
    @Override
    public int getSize() {
        return size;
    }
    @Override
    public boolean isEmpty() {
        return front == tail;
    }
    @Override
    public void enqueue(E e) {
        if ((tail + 1) % data.length == front) {
            resize(getCapacity() * 2);
        }
        data[tail] = e;
        tail = (tail + 1) % data.length;
        size ++;
    }
    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 E dequeue() {
        if (isEmpty()) {
            throw new NoSuchElementException("Cannot dequeue from an empty queue!");
        }
        E ret = data[front];
        data[front] = null;
        front = (front + 1) % data.length;
        size --;
        // 缩容
        if (size == getCapacity() / 4 && getCapacity() / 2 != 0){
            resize(getCapacity() / 2);
        }
        return ret;
    }
    @Override
    public E getFront() {
        if (isEmpty()) {
            throw new NoSuchElementException("Cannot dequeue from an empty queue!");
        }
        return data[front];
    }
    @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<>();
        for (int i = 0; i < 10; i++) {
            queue.enqueue(i);
            System.out.println(queue);
            if (i % 3 == 2) {
                queue.dequeue();
                System.out.println(queue);
            }
        }
    }
}