栈和队列

import java.util.InputMismatchException;

class Node{
    int val;
    Node next;

    public Node(int val, Node next) {
        this.val = val;
        this.next = next;
    }
    public Node(int val){
        this(val,null);
    }
}
//插入元素
public class MyQueue {
    private Node head=null;
    private Node tail=null;
    private int size=0;

    public void offer(int v){
        Node node=new Node (v);
        if(tail==null){
            head=node;
        }
        tail.next=node;
        tail.next=tail;
        size++;
    }
    //删除元素
    public Integer poll(){
        if(head==null){
            return null;
        }
       int val=head.val;
        head=head.next;
        if(head==null){
            return null;
        }
        return val;

    }
    //获取首元素
    public Integer peek(){
        if(head==null){
            return null;
        }
        return head.val;
    }

//栈的使用
//规则是后进先出

public class MyStack {
    private int[] arr=new int[100];
    private int size=0;
    //插入元素
    public void push(int v){
        arr[size++]=v;
    }
    //删除元素
    public int pop(){
        return arr[size--];
    }
    //获取删除元素的值
    public int peek(){
        return arr[size-1];
    }
}

/循环队列
public class MyQueueByArray {
    private int[] array=new int[100];
    private int head=0;
    private int tail=0;
    private int size=0;
//元素入列
    public void offer(int v){
        if(size==array.length){
            return ;
        }
        array[tail]=v;
        tail++;
        if(tail>=array.length){
                tail=0;
        }
        size++;
    }
   //元素出列
    public Integer poll(){
        if(size==0){
            return null;
        }
        int val=array[head];
        head++;
        if(head>=array.length){
            head=0;
        }
        size--;
        return val;
    }
   //获取首元素
    public Integer peek(){
        if(size==0){
            return null;
        }
        return array[head];
    }

}