队列的模拟及环形队列思路

定义

队列是一个有序列表，可以用数组或是链表来实现。
遵循先入先出的原则。即：先存入队列的数据，要先取出。后存入的要后取出

模拟思路

队列本身是有序列表，若使用数组的结构来存储队列的数据，则队列数组的声明如下图, 其中 maxSize 是该队列的最大容量
因为队列的输出、输入是分别从前后端来处理，因此需要两个变量 front及 rear分别记录队列前后端的下标，front 会随着数据输出而改变，而 rear则是随着数据输入而改变，如图所示

入队出队操作模拟

当我们将数据存入队列时称为”addQueue”，addQueue 的处理需要有两个步骤：

将尾指针往后移：rear+1 , 当 front == rear 时，队列为空
若尾指针 rear 小于队列的最大下标 maxSize-1，则将数据存入 rear所指的数组元素中，否则无法存入数据。rear == maxSize - 1时，队列满

注意：front指向的是队列首元素的前一个位置

实现代码

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import java.util.Scanner;
  
public class ArrayQueueDemo {
    public static void main(String[] args) {
        ArrayQueue queue = new ArrayQueue(3);
        Scanner scanner = new Scanner(System.in);
        boolean flag = true;
        while (flag){
            System.out.println("输入a(add)添加数据");
            System.out.println("输入g(get)取出数据");
            System.out.println("输入s(show)显示所有数据");
            System.out.println("输入h(head)显示头部数据");
            System.out.println("输入e(exit)退出程序");
            char c = scanner.next().charAt(0);
            switch (c){
                case 'a':
                    System.out.println("请输入数据");
                    int num = scanner.nextInt();
                    queue.addNum(num);
                    break;
                case 'g':
                    try {
                        queue.getQueue();
                        System.out.println("取出成功");
                    }catch (Exception e){
                        System.out.println(e.getMessage());
                    }
                    break;
                case 's':
                    queue.showQueue();
                    break;
                case 'h':
                    try {
                        queue.headQueue();
                    }catch (Exception e){
                        System.out.println(e.getMessage());
                    }
                    break;
                case 'e':
                    flag = false;
                    System.out.println("程序已退出");
                    break;
                default:
                    break;
            }
        }
        scanner.close();
    }
}
  
class ArrayQueue{
    private int maxSize;//队列的大小
    private int front;//指向队列首元素的前一个位置
    private int rear;//指向队列的尾元素
    private int[] arr;//用数组来实现队列
  
    //构造方法初始化
    public ArrayQueue(int maxSize) {
        this.maxSize = maxSize;
        front = -1;//指向的是队列第一个元素的前一个位置
        rear = -1;
        arr = new int[maxSize];
    }
  
    /**
     * 判断队列是否装满
     * @return
     */
    public boolean isFull(){
        return rear == maxSize-1;
    }
  
    /**
     * 判断队列是否为空
     * @return
     */
    public boolean isEmpty(){
        return front == rear;
    }
  
    /**
     * 为队列添加数据
     * @param num
     */
    public void addNum(int num){
        if (isFull()){
            System.out.println("队列已经满了,无法再加入数据");
            return;
        }
        rear++;
        arr[rear] = num;
    }
  
    /**
     * 取出队列中的数据
     * @return
     */
    public int getQueue(){
        if (isEmpty()){
            throw new RuntimeException("队列为空,不能取出数据");
        }
        front++;
        return arr[front]=0;
    }
  
    /**
     * 显示队列的所有数据
     */
    public void showQueue(){
        if (isEmpty()) {
            System.out.println("队列为空,没有数据");
            return;
        }
        for (int i = 0; i<arr.length; i++){
            System.out.printf("arr[%d]=%d\n",i,arr[i]);
        }
    }
  
    /**
     * 显示队列的头数据,注意不是取出数据
     * @return
     */
    public int headQueue(){
        if (isEmpty()){
            throw new RuntimeException("队列为空,没有数据");
        }
        return arr[front+1];
    }
}

　　运行结果

注意：因先入先出的原则，front和rear最终都会在队列顶部，所以上述队列只能一次性使用，没有达到复用的效果，因此我们要用到环形队列

环形队列

思路：

front变量含义调整：front变量指向队首元素，初值为0
rear变量含义调整：rear变量指向队尾元素的下一个元素，初值为0。规定空出一个位置
队列为空的判定条件：front == rear
队列为满的判定条件：(rear + 1) % maxSize == front
队列中有效元素的个数：(rear - front + maxSize) % maxSize
入队和出队时，都需要让标记对maxSize取模

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import java.util.Scanner;
  
public class CyclicArrayQueueDemo {
    public static void main(String[] args) {
        CyclicArrayQueue queue = new CyclicArrayQueue(3);
        Scanner scanner = new Scanner(System.in);
        boolean flag = true;
        while (flag){
            System.out.println("输入a(add)添加数据");
            System.out.println("输入g(get)取出数据");
            System.out.println("输入s(show)显示所有数据");
            System.out.println("输入h(head)显示头部数据");
            System.out.println("输入e(exit)退出程序");
            char c = scanner.next().charAt(0);
            switch (c){
                case 'a':
                    System.out.println("请输入数据");
                    int num = scanner.nextInt();
                    queue.addNum(num);
                    break;
                case 'g':
                    try {
                        int n = queue.getQueue();
                        System.out.println("取出的数是:"+n);
                    }catch (Exception e){
                        System.out.println(e.getMessage());
                    }
                    break;
                case 's':
                    queue.showQueue();
                    break;
                case 'h':
                    try {
                        queue.headQueue();
                    }catch (Exception e){
                        System.out.println(e.getMessage());
                    }
                    break;
                case 'e':
                    flag = false;
                    System.out.println("程序已退出");
                    break;
                default:
                    break;
            }
        }
        scanner.close();
    }
}
  
class CyclicArrayQueue {
    private int maxSize;//队列的大小
    private int front;//front变量指向队首元素，初值为0
    private int rear;//rear变量指向队尾元素的下一个元素，初值为0。规定空出一个位置
    private int[] arr;//用数组来实现队列
  
    public CyclicArrayQueue(int maxSize) {
        this.maxSize = maxSize;
        arr = new int[maxSize];
    }
  
    /**
     * 判断队列是否装满
     *
     * @return
     */
    public boolean isFull() {
        return (rear + 1) % maxSize == front;
    }
  
    /**
     * 判断队列是否为空
     *
     * @return
     */
    public boolean isEmpty() {
        return front == rear;
    }
  
    /**
     * 为队列添加数据
     *
     * @param num
     */
    public void addNum(int num) {
        if (isFull()) {
            System.out.println("队列已经满了,无法再加入数据");
            return;
        }
        arr[rear] = num;
        rear = (rear + 1) % maxSize;
    }
  
    /**
     * 取出队列中的数据
     *
     * @return
     */
    public int getQueue() {
        if (isEmpty()) {
            throw new RuntimeException("队列为空,不能取出数据");
        }
        int value = arr[front];
        front = (front + 1) % maxSize;
        return value;
    }
  
    /**
     * 显示队列的所有数据
     */
    public void showQueue() {
        if (isEmpty()) {
            System.out.println("队列为空,没有数据");
            return;
        }
        for (int i = front; i < front + size(); i++) {
            System.out.printf("arr[%d]=%d\n", i % maxSize, arr[i % maxSize]);
        }
    }
  
    public int size() {
        return (rear + maxSize - front) % maxSize;
    }
  
    /**
     * 显示队列的头数据,注意不是取出数据
     * @return
     */
    public int headQueue(){
        if (isEmpty()){
            throw new RuntimeException("队列为空,没有数据");
        }
        return arr[front];
    }
}