队列

队列和栈刚好相反啦，是先进先出的线性表
它只能在表的一端进行插入，另一端进行删除元素
允许插入的一端叫做队尾，允许删除的一端叫做队头

循环队列

class QueueDemo{
   private int front;
   private int rear;
   private int[] elem;
   private int usedSize = 0;
   private int allSize = 10;

   public QueueDemo(){
       this(10);
   }

   public QueueDemo(int size){
       this.front = 0;
       this.rear = 0;
       this.elem = new int[size];
   }

   
  //入队
    public void push(int val){
       if (isFull()){
           return;
       }
       this.elem[this.rear] = val;
       this.rear = (this.rear+1) % allSize;
       usedSize++;
    }
    //出队
    public int pop(){
       if (isEmpty()){
           throw new UnsupportedOperationException("队列为空");
       }
       int num = this.elem[front];
       this.elem[this.front] = -1;  //如果对列里面是引用，这里置为null
       this.front = (this.front+1) % allSize;
       this.usedSize--;
       return num;
    }

    //判空
    public boolean isEmpty(){
       if (this.rear == this.front){
           return true;
       }
       return false;
    }

    //判满
    public boolean isFull(){
       if ((this.rear + 1) % allSize == this.front){
           return true;
       }
       return false;
    }
    //打印
    public void show(){
        for (int i = this.front; i != this.rear; i = (i+1) % allSize) {
            System.out.printf(this.elem[i]+" ");
        }
        System.out.println();
    }
}

测试

public class TestQueueDemo {
    public static void main(String[] args) {
        QueueDemo queueDemo = new QueueDemo();
        for (int i = 0; i < 8; i++) {
            queueDemo.push(i);
        }
        queueDemo.show();
        queueDemo.pop();
        queueDemo.show();
    }
}

链式队列

class LinkQueue{
    class Entry{
        int data;
        Entry next;
        public Entry(){
            this.data = -1;
            this.next = null;
        }
        public Entry(int val){
            this.data = val;
            this.next = null;
        }
    }

    private Entry front; //头结点
    private Entry rear; //尾结点
    private int usedSize; //用过的大小

    public  LinkQueue(){
        this.front = null;
        this.rear = null;
        this.usedSize = 0;
    }
   //入队
   public void push(int val){
       Entry entry = new Entry(val);
        if (isEmpty()){ //空的话
            this.front = entry; //队尾和队头都指向entry
            this.rear = this.front;
        }else { //非空
            this.rear.next = entry; //队尾插入
            this.rear = this.rear.next; //队尾向后走
        }
       this.usedSize++; //用过的大小加一
   }
   //出队
    public int pop(){
        if (isEmpty()){ 
            throw new UnsupportedOperationException("队列为空");
        }
        Entry delete = this.front; //队头出队
        this.front = delete.next; //front后移
        this.usedSize--; //用过的大小减一
        return delete.data; 

    }
    //判空
    public boolean isEmpty(){
        if (this.usedSize == 0){
            return true;
        }
        return false;
    }
    //打印
    public void show(){
        Entry cur  = this.front;
        while (cur != null){
            System.out.print(cur.data+" ");
            cur = cur.next;
        }
        System.out.println();
    }
    
}

测试

public class LinkQueueDemo {
    public static void main(String[] args) {
        LinkQueue linkQueue = new LinkQueue();
        for (int i = 0; i < 10; i++) {
            linkQueue.push(i);
        }
        linkQueue.show();
        linkQueue.pop();
        linkQueue.show();
    }
}