用双链表模仿双端队列

知己知彼，百战不殆，那么双端队列的特色是啥：（1）头部添加，头部取 （2）尾部添加，尾部取 （3）头部添加，尾部取 （4）尾部添加，头部取 总之就是想怎么加就怎么加，想怎么取就怎么取

/**
 * 用双链表双端队列
 */
public class Code04 {
    public static class Node<V> {
        public V value;
        public Node<V> last;
        public Node<V> next;

        public Node(V v) {
            value = v;
            last = null;
            next = null;
        }
    }

    public static class MyQueue<V> {
        private Integer size;
        private Node<V> head;
        private Node<V> tail;
        public MyQueue() {
            size = 0;
        }

        public MyQueue(Integer size) {
            this.size = size;
        }

        public boolean isEmpty(){
            return size==0;
        }

        public Node pushHead(Integer value){
            Node add = new Node(value);
            if(size == 0){
                head = add;
                tail = add;
            } else {
                head.last=add;
                add.next=head;
                head=head.last;
            }
            size++;
            return head;
        }

        public Node pushTail(Integer value){
            Node add = new Node(value);
            if(size == 0){
                head = add;
                tail = add;
            } else {
                tail.next=add;
                add.last=tail;
                tail=tail.next;
            }
            size++;
            return head;
        }

        public Integer popHead(){
            if(isEmpty()){
                return null;
            }
            Integer value = (Integer) head.value;
            if(head==tail){
                head=null;
                tail=null;
            }else{
                head = head.next;
                head.last=null;
            }
            size--;
            return value;
        }

        public Integer popTail(){
            if(isEmpty()){
                return null;
            }
            Integer value = (Integer) tail.value;
            if(head==tail){
                tail=null;
                head=null;
            }else {
                tail = tail.last;
                tail.next=null;
            }
            size--;
            return value;
        }

        public Integer peek(){
            if(isEmpty()){
                return null;
            }
            return (Integer) head.value;
        }

    }

    public static void main(String[] args) {
        MyQueue stack = new MyQueue();
//        stack.pushHead(1);
//        stack.pushHead(2);
//        stack.pushHead(3);
//
//        System.out.println(stack.popHead());
//        System.out.println(stack.popHead());
//        System.out.println(stack.popHead());
//        System.out.println(stack.popHead());

        stack.pushTail(1);
        stack.pushTail(2);
        stack.pushTail(3);

        System.out.println(stack.popHead());
        System.out.println(stack.popTail());
        System.out.println(stack.popHead());
        System.out.println(stack.popHead());

    }
}