队列

基本介绍

1. 队列是一个有序列表，可以用数组或是链表来实现。
2. 遵循先入先出原则。
3. 示意图：（使用数组模拟队列示意图）
   

数组模拟队列、

思路

• 如下图：front指向队列头部即指向队列头的前一个位置，rear指向队尾即指向队列尾的数据，maxSize是队列的最大容量。front随着数据输出而改变，rear随着数据输入而改变。
• front和rear的初始值为-1。
• 空条件：front == rear
• 满条件：rear == maxSize - 1
  

代码

package com.queue;

import java.util.Scanner;

public class ArrayQueueDemo {
    public static void main(String[] args) {
        System.out.println("~~~~~~~~~~~~~~~~~~~");
        System.out.println("a -> 添加");
        System.out.println("g -> 获取");
        System.out.println("s -> 显示队列");
        System.out.println("h -> 显示队头");
        System.out.println("e -> 退出");
        System.out.println("~~~~~~~~~~~~~~~~~~~");
        ArrayQueue arrayQueue = new ArrayQueue(3);
        char c = ' ';
        boolean loop = true;
        Scanner scanner = new Scanner(System.in);
        while(loop){
            System.out.println("请输入操作选项：");
            c = scanner.next().charAt(0);
            switch (c){
                case 'a':
                    System.out.println("请输入添加的数据：");
                    int add = scanner.nextInt();
                    arrayQueue.addQueue(add);
                    break;
                case 'g':
                    try {
                        arrayQueue.getQueue();
                    }catch (RuntimeException e){
                        System.out.println(e.getMessage());
                    }
                    break;
                case 's':
                    arrayQueue.showQueue();
                    break;
                case 'h':
                    try {
                        arrayQueue.showHead();
                    }catch (RuntimeException e){
                        System.out.println(e.getMessage());
                    }
                    break;
                case 'e':
                    loop = false;
                    break;
                default:
                    break;
            }
        }
    }
}
class ArrayQueue{
    private int maxSize;//数组最大值
    private int front;//队首
    private int rear;//队尾
    private int[] arr;//存放数据的数组


    public ArrayQueue(int maxSize){
        this.maxSize = maxSize;
        arr = new int[maxSize];
        this.front = -1;
        this.rear = -1;
    }

    /**
     * 判断队列是否满
     * @return
     */
    public boolean isFull(){
        return this.rear == maxSize - 1;
    }

    /**
     * 判断队列是否为空
     * @return
     */
    public boolean isEmpty(){
        return this.front == this.rear;
    }

    /**
     * 向队列中插入元素
     * @param n
     */
    public void addQueue(int n){
        if(isFull()){
            System.out.println("队列已经满，不能插入~~");
            return;
        }
        arr[++this.rear] = n;
    }

    /**
     * 获取队列元素
     * @return
     */
    public int getQueue(){
        if(isEmpty()){
            throw new RuntimeException("队列为空~~");
        }
        return arr[++this.front];
    }

    /**
     * 显示队列
     */
    public void showQueue(){
        if(isEmpty()){
            System.out.println("队列为空！");
            return;
        }
        for (int i = 0; i < arr.length; i++) {
            System.out.printf("arr[%d]=%d\n",i,arr[i]);
        }
    }

    /**
     * 显示队首
     */
    public void  showHead(){
        if(isEmpty()){
            throw new RuntimeException("队列为空！");
        }
        System.out.println("队首为："+arr[this.front+1]);
    }
}

缺点

数组只能使用一次。无法达到复用效果。

数组模拟环形队列

思路

• 将数组看作环形，通过取模的方式实现。
• front指向队列的第一个元素。
• rear指向队列的最后一个元素的后一个位置。
• front 和 rear 初始值都为 0。
• 有效数据个数： (rear - front + maxSize) % maxSize
• 空条件：(rear + 1) % maxSize == front
• 满条件：rear == front

代码

package com.queue;

import java.util.Scanner;

public class CircleArrayQueueDemo {
    public static void main(String[] args) {
        System.out.println("~~~~~~~~~~~~~~~~~~~");
        System.out.println("a -> 添加");
        System.out.println("g -> 获取");
        System.out.println("s -> 显示队列");
        System.out.println("h -> 显示队头");
        System.out.println("e -> 退出");
        System.out.println("~~~~~~~~~~~~~~~~~~~");
        CircleArrayQueue circleArrayQueue = new CircleArrayQueue(4);
        char c = ' ';
        boolean loop = true;
        Scanner scanner = new Scanner(System.in);
        while(loop){
            System.out.println("请输入操作选项：");
            c = scanner.next().charAt(0);
            switch (c){
                case 'a':
                    System.out.println("请输入添加的数据：");
                    int add = scanner.nextInt();
                    circleArrayQueue.addQueue(add);
                    break;
                case 'g':
                    try {
                        int queue = circleArrayQueue.getQueue();
                        System.out.println("取走："+queue);
                    }catch (RuntimeException e){
                        System.out.println(e.getMessage());
                    }
                    break;
                case 's':
                    circleArrayQueue.showQueue();
                    break;
                case 'h':
                    try {
                        circleArrayQueue.showHead();
                    }catch (RuntimeException e){
                        System.out.println(e.getMessage());
                    }
                    break;
                case 'e':
                    loop = false;
                    break;
                default:
                    break;
            }
        }
    }
}

class CircleArrayQueue{
    private int maxSize;
    private int front;
    private int rear;
    private int[] arr;


    public CircleArrayQueue(int maxSize){
        this.maxSize = maxSize;
        arr = new int[maxSize];
    }


    public boolean isFull(){
        return (this.rear + 1) % maxSize == this.front;
    }

    public boolean isEmpty(){
        return this.front == this.rear;
    }

    public void addQueue(int n){
        if(isFull()){
            System.out.println("队列已经满，不能插入~~");
            return;
        }
        arr[this.rear] = n;
        this.rear = (this.rear + 1) % maxSize;
    }

    public int getQueue(){
        if(isEmpty()){
            throw new RuntimeException("队列为空~~");
        }
        int value =  arr[this.front];
        this.front = (this.front + 1) % maxSize;
        return value;
    }

    public void showQueue(){
        if(isEmpty()){
            System.out.println("队列为空！");
            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;
    }

    public void  showHead(){
        if(isEmpty()){
            throw new RuntimeException("队列为空！");
        }
        System.out.println("队首为："+arr[this.front]);
    }
}