栈和队列

public class Queue {

    private Element head;
    private Element tail;
    public int size;

    public void enqueue(Element e){
        if(tail == null){
            head = e;
            tail = e;
        }else{
            tail.next = e;
            tail = e;
        }
        size++;
    }
    public Element dequeue(){
        if(head == null){
            return null;
        }else{
            Element p = head;
            head = head.next;
            p.next = null;
            if(head == null){
                tail = null;
            }
            size--;
            return p;
        }
    }
}

public class Stack {

    public Element top;
    public int sum;

    public Element pop(){
        if(top == null){
            return null;
        }else{
            Element p = top;
            sum -= top.value;
            top = top.next;
            p.next = null;
            return p;
        }

    }

    public void push(Element e){
        if(top == null){
            top = e;
        }else{
            e.next = top;
            top = e;
        }
        sum += e.value;
    }

}