13-循环双端队列

//循环双端队列
package com.weyan.circle;
@SuppressWarnings("unchecked")

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];
    }
    
    //元素个数
    public int size() {
        return size;
    }
    
    //队列是否为空
    public boolean isEmpty() {
        return size == 0;
    }
    
    //从队尾入队
    public void enQueueRear(E element) {
        ensureCapacity(size + 1);
        elements[index(size)] = element;
        size ++;
    }
    
    //从队尾出队
    public E deQueueRear() {
        int rearIndex = index(size - 1);
        E rear = elements[rearIndex];
        elements[rearIndex] = null;
        size -- ;
        return rear;
    }
    
    //从队头入队
    public void enQueueFront(E element) {
        ensureCapacity(size + 1);
        front = index(-1);
        elements[front] = element;
        size ++;
    }
    
    //从队头出队
    public E deQueueFront() {
        E frontElement = elements[front];
        elements[front] = null;
        front = index(1);
        size --;
        return frontElement;
    }
    
    //队头元素
    public E front() {
        return elements[front];
    }
    
    //队尾元素
    public E rear() {
        return elements[index(size - 1)];
    }
    
    public int index(int index) {
        index += front;
        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;
        
        // 新容量为旧容量的1.5倍
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        E[] newElements = (E[]) new Object[newCapacity];
        for (int i = 0; i < size; i++) {
            newElements[i] = elements[(i+front) % elements.length];
        }
        elements = newElements;
        //重置front
        front = 0;
        System.out.println(oldCapacity + "扩容为" + newCapacity);
    }
    
    @Override
    public String toString() {
        // TODO Auto-generated method stub
        StringBuilder string = new StringBuilder();
        string.append("capacity=").append(elements.length)
        .append(",size=").append(size).append(",front=").append(front).append(",[");
        for (int i = 0; i < elements.length; i++) {
            if (i != 0) {
                string.append(", ");
            }
            string.append(elements[i]);
        }
        string.append("]");
        return string.toString();
    }
}

验证结果：

//测试循环双端队列
    static void test3() {
        CircleDeque<Integer> queue = new CircleDeque<Integer>();
        //头5 4 3 2 1 100 101 102 103 104 105 106 8 7 6 尾
        
        //头8 7 6 5 4 3 2 1 100 101 102 103 104 105 106 107 108 109 null null 10 9 尾
        for (int i = 0; i < 10; i++) {
            queue.enQueueFront(i + 1);
            queue.enQueueRear(i + 100);
        }
        
        System.out.println(queue);
                while (!queue.isEmpty()) {
            System.out.println(queue.deQueueFront());
        }
    }
-------------------------------------------------------------------------------------------------------
10扩容为15
15扩容为22
capacity=22,size=20,front=20,[8, 7, 6, 5, 4, 3, 2, 1, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, null, null, 10, 9]
10
9
8
7
6
5
4
3
2
1
100
101
102
103
104
105
106
107
108
109

测试2：

//测试循环双端队列
    static void test3() {
        CircleDeque<Integer> queue = new CircleDeque<Integer>();
        //头5 4 3 2 1 100 101 102 103 104 105 106 8 7 6 尾
        
        //头8 7 6 5 4 3 2 1 100 101 102 103 104 105 106 107 108 109 null null 10 9 尾
        for (int i = 0; i < 10; i++) {
            queue.enQueueFront(i + 1);
            queue.enQueueRear(i + 100);
        }
        
        //头null 7 6 5 4 3 2 1 100 101 102 103 104 105 106 null null null null null null null 尾
        for (int i = 0; i < 3; i++) {
            queue.deQueueFront();
            queue.deQueueRear();
        }
        
        //头11 7 6 5 4 3 2 1 100 101 102 103 104 105 106 null null null null null null 12 尾
        queue.enQueueFront(11);
        queue.enQueueFront(12);
        System.out.println(queue);
        while (!queue.isEmpty()) {
            System.out.println(queue.deQueueFront());
        }
    }
----------------------------------------------------------------------------------------------------------
10扩容为15
15扩容为22
capacity=22,size=16,front=21,[11, 7, 6, 5, 4, 3, 2, 1, 100, 101, 102, 103, 104, 105, 106, null, null, null, null, null, null, 12]
12
11
7
6
5
4
3
2
1
100
101
102
103
104
105
106

总结：

用动态数组实现循环队列和循环双端队列的目的：是对动态数组的优化和另外一种思想（思路）实现队列。