队列 Queue

概述

队列(Queue)是一个先进先出(FIFO：First In First Out)的有序表，可以由数组或者链表实现。
由链表实现的队列和List的区别在于，List可以在任意位置添加和删除元素，
而队列只有两个操作：1.把元素添加到队列末尾；2.从队列头部取出元素。

数组队列

和普通数组的区别在于，array可以在任意位置添加和删除元素，
而队列只有两个操作：1.把元素添加到队列末尾；2.从队列头部取出元素。

思路

1. 初始化:front,rear,max size
2. isFull: rear == maxSize -1，isEmpty: rear == front
3. addQueue：rear 后移,getQueue：front后移
4. viewQueue，headQueue: 查看头部

缺点

该数组只能用一次，取出数据后空间不可用。可以改进成环形数组。

造轮子

public class ArrayQueue {
    private int maxSize;//数组最大容量
    private int front;//队列头部
    private int rear;//队列尾部
    private int[] arr;//存放数据模拟队列

    //初始化
    public ArrayQueue(int maxSize){
        this.maxSize = maxSize;
        arr = new int[maxSize];
        front = -1;//队列头部前一个位置
        rear = -1;//队列尾部
    }

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

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

    //添加值
    public void addQueue(int n){
        if(isFull()){
            System.out.println("no available space in queue ");
            return;
        }
        rear++;
        arr[rear] = n;
    }

    //取值
    public int getQueue(){
        if(isEmpty()){
            throw new RuntimeException("queue empty");
        }
        front++;
        return arr[front];
    }

    //显示队列
    public void viewQueue(){
        if(isEmpty()){
            System.out.println("empty queue");
            return;
        }
        for(int x : arr){
            System.out.println(x);
        }
    }

    //查看头部
    public int headQueue(){
        if(isEmpty()){
            throw new RuntimeException("empty queue");
        }
        return arr[front + 1];
    }
}

环形数组队列

实际上并没有实际的环形，对代码进行处理便可以实现虚拟环形。

思路

1. front 指向队列第一个元素，初始值为0。
2. rear 指向队列最后一个元素的后一个位置，预留一个位置作判断，初始值为0。
3. (rear + 1)%maxSize == front ，队列为满.利用remainder查看虚拟环形下一个位置。

remainder 实际作用是让指针在规定区间移动，达到虚拟环绕效果
4. rear = front ,队列为空
5. 有效数据个数，(rear-front+maxSize)%maxSize

造轮子

public class CircleArrayQueue {
    private int maxSize;//数组最大容量
    private int front;//队列头部
    private int rear;//队列尾部
    private int[] arr;//存放数据模拟队列

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

    //判断队列是否满
    public boolean isFull(){
        return (rear + 1)%maxSize == front;
    }

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

    //添加值
    public void addQueue(int n){
        if(isFull()){
            System.out.println("no available space in queue ");
            return;
        }
        arr[rear] = n;
        //remainder,超出范围往前跳
        rear = (rear + 1) % maxSize;
    }

    //取值
    public int getQueue(){
        if(isEmpty()){
            throw new RuntimeException("queue empty");
        }
        int temp = arr[front];
        front = (front + 1)%maxSize;
        return temp;
    }

    //显示队列
    public void viewQueue(){
        if(isEmpty()){
            System.out.println("empty queue");
            return;
        }
        for(int i = front;i<front+size();i++){
            System.out.println("arr["+i%maxSize+"]= "+arr[i%maxSize]  );
        }
    }

    public int size(){
        return (rear - front +maxSize)%maxSize;
    }

    //查看头部
    public int headQueue(){
        if(isEmpty()){
            throw new RuntimeException("empty queue");
        }
        return arr[front];
    }

链表队列

// A linked list (LL) node to store a queue entry 
class QNode { 
    int key; 
    QNode next; 
  
    // constructor to create a new linked list node 
    public QNode(int key) 
    { 
        this.key = key; 
        this.next = null; 
    } 
} 
  
// A class to represent a queue 
// The queue, front stores the front node of LL and rear stores the 
// last node of LL 
class Queue { 
    QNode front, rear; 
  
    public Queue() 
    { 
        this.front = this.rear = null; 
    } 
  
    // Method to add an key to the queue. 
    void enqueue(int key) 
    { 
  
        // Create a new LL node 
        QNode temp = new QNode(key); 
  
        // If queue is empty, then new node is front and rear both 
        if (this.rear == null) { 
            this.front = this.rear = temp; 
            return; 
        } 
  
        // Add the new node at the end of queue and change rear 
        this.rear.next = temp; 
        this.rear = temp; 
    } 
  
    // Method to remove an key from queue. 
    void dequeue() 
    { 
        // If queue is empty, return NULL. 
        if (this.front == null) 
            return; 
  
        // Store previous front and move front one node ahead 
        QNode temp = this.front; 
        this.front = this.front.next; 
  
        // If front becomes NULL, then change rear also as NULL 
        if (this.front == null) 
            this.rear = null; 
    } 
}