队列 - 双端队列 - 循环队列 - 循环双端队列

队列是一种特殊的线性表，只能在头尾两端进行操作
队尾(rear):只能从队尾添加元素，一般叫做 enQueue，入队
队头(front):只能从队头移除元素，一般叫做 deQueue，出队
先进先出的原则，First In First Out，FIFO

普通队列 - java实现

public class Queue<E> {
    private LinkedList<E> list = new LinkedList<>();

    public int size() {
        return list.size();
    }

    public boolean isEmpty() {
        return list.isEmpty();
    }

    public void clear() {
        list.clear();
    }

    // 入队
    public void enQueue(E element) {
        list.add(list.size() - 1, element);
    }

    // 出队
    public E deQueue() {
        return list.remove(0);
    }

    // 获取队列的头元素
    public E front() {
        return list.get(0);
    }
}

双端队列 - java实现

public class Deque <E> {
    private LinkedList<E> list = new LinkedList<>();

    public int size() {
        return list.size();
    }

    public boolean isEmpty() {
        return list.isEmpty();
    }

    public void clear() {
        list.clear();
    }

    // 队尾入队
    public void enQueueRear(E element) {
        list.add(list.size()- 1, element);
    }

    // 从队头出队
    public E deQueueFront() {
        return list.remove(0);
    }

    // 从队头入队
    public void enQueueFront(E element) {
        list.add(0, element);
    }

    // 从队尾出队
    public E deQueueRear() {
        return list.remove(list.size()-1);
    }

    // 获得队头元素
    public E front() {
        return list.get(0);
    }

    // 获得队尾元素
    public E rear() {
        return list.peekLast();
    }
}

循环队列 - java实现

public class CircleQueue <E>{

    private int front;
    private int size;
    private E[] elements;
    private static final int DEFAULT_CAPACITY = 10;

    public CircleQueue() {
        elements = (E[]) new Object[DEFAULT_CAPACITY];
    }

    public int size() {
        return size;
    }

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

    public void clear() {
        for (int i = 0; i < size; i++) {
            elements[index(i)] = null;
        }
        front = 0;
        size = 0;
    }

    // 入队
    public void enQueue(E element) {
        ensureCapacity(size + 1);
        elements[index(size)] = element;
        size++ ;
    }

    // 出队
    public E deQueue() {
        E frontElement = elements[front];
        elements[front] = null;
        front = index(1);
        size-- ;
        return frontElement;
    }

    // 队列头元素
    public E front() {
        return elements[front];
    }

    /***
     *  索引 0  1   2   3   4   5   6
     *  元素       11   22  33  44  55
     *  此时如果往 [11, 22, 33, 44, 55] 后面加元素 应该加到 索引为 0  的位置
     *  需要进行索引转换
     * @return
     */
    private int index(int index) {
        return (front + index) % elements.length;
    }

    /**
     * 保证要有capacity的容量
     * @param capacity
     */
    private void ensureCapacity(int capacity) {
        int oldCapacity = elements.length;
        if (oldCapacity >= capacity) return;
        // 扩容
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        E[] newElements = (E[]) new Object();
        for (int i = 0; i < elements.length; i++) {
            newElements[i] = elements[i];
        }

        elements = newElements;
        // 重置头节点
        front = 0;
    }
}

循环双端队列 - java实现

/**
 * 循环双端队列
 */
public class CircleDeque<E> {

    private int front;
    private int size;
    private E[] elements;
    private static final int DEFAULT_CAPACITY = 10;

    public CircleDeque() {
        elements = (E[]) new Object[DEFAULT_CAPACITY];
    }

    public int size() {
        return size;
    }

    public void clear() {
        for (int i = 0; i < size; i++) {
            elements[i] = null;
        }
        size = 0;
        front = 0;
    }

    /**
     * 从尾部入队
     * @param element
     */
    public void enQueueRear(E element) {
        ensureCapacity(size + 1);

        elements[index(size)] = element;
        size++ ;
    }

    /**
     * 从头部出队
     */
    public E deQueueFront() {
        E first = elements[front];
        elements[front] = null;
        front = index(1);
        size-- ;
        return first;
    }

    /**
     * 从头部入队
     * @param element
     */
    public void enQueueFront(E element) {
        ensureCapacity(size + 1);
        front = index(-1);
        elements[front] = element;
        size++;
    }

    /**
     * 从尾部出队
     */
    public E deQueueRear() {
        E lastElement = elements[index(size - 1)];
        elements[index(size - 1)] = null;
        size-- ;
        return lastElement;
    }

    public E front() {
        return elements[front];
    }

    public E rear() {
        return elements[index(size - 1)];
    }

    private int index(int index) {
        index += front;
        //
        /***
         * 考虑这种情况
         *  索引 0   1    2    3    4     5   6
         *  元素 11  22   33   44   55    66
         *  往头部入队的时候 front = 0 ; index(-1) + front = -1 < 0
         */
        if (index < 0) return index + elements.length;
        return index % elements.length;
    }

    /**
     * 保证要有capacity的容量
     * @param capacity
     */
    private void ensureCapacity(int capacity) {
        int oldCapacity = elements.length;
        if (oldCapacity >= capacity) return;
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        E[] newElements = (E[]) new Object[newCapacity];
        for (int i = 0; i < oldCapacity; i++) {
            newElements[i] = elements[i];
        }
        elements = newElements;

        // 重置头节点位置
        front = 0;
    }
}