用双链表实现双端队列

//双链表
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;
    }
}
//双端队列 前后皆可进出
public static class MyDeque<V>{
    private Node<V> head;
    private Node<V> tail;
    private int size;

    public MyDeque(){
        head = null;
        tail = null;
        size = 0;
    }

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

    public int size(){
        return size;
    }
	//从头进
    public void pushHead(V value){
        Node<V> cur = new Node<>(value);
        if (head == null){
            head = cur;
            tail = cur;
        }else{
            cur.next = head;
            head.last = cur;
            head = cur;
        }
        size++;
    }
	//从尾进
    public void pushTail(V value){
        Node<V> cur = new Node<>(value);
        if (head == null){
            head = cur;
            tail = cur;
        }else{
            tail.next = cur;
            cur.last = tail;
            tail = cur;
        }
        size++;
    }
	//从头出
    public V pollHead(){
        V ans = null;
        if (head == null)
            return null;
        size--;
        ans = head.value;
        if (head == tail){
            head = null;
            tail = null;
        }else{
            head = head.next;
            head.last = null;
        }
        return ans;
    }
	//从尾出
    public V pollTail(){
        V ans = null;
        if (head == null)
            return null;
        size--;
        ans = tail.value;
        if (head == tail){
            head = null;
            tail = null;
        }else{
            tail = tail.last;
            tail.next = null;
        }
        return ans;
    }
	//从头查
    public V peekHead(){
        if (head != null)
            return head.value;
        else
            return null;
    }
	//从尾查
    public V peekTail(){
        if (head != null)
            return tail.value;
        else
            return null;
    }
}