双向链表实现队列和栈 O(1)

public class MyStackAndQueue {
    public static class Node<T> {
        public T value;
        public Node<T> next;
        public Node<T> last;

        public Node(T data) {
            value = data;
        }
    }
    public static class DoubleEndsQueue<T> {
        public Node<T> head;
        public Node<T> tail;

        public void addFormHead(T value) {
            Node<T> cur = new Node<>(value);
            if (head == null) {
                head = cur;
                tail = cur;
            } else {
                cur.next = head;
                head.last = cur;
                head = cur;
            }
        }
        
        public void addFormTail(T value) {
            Node<T> cur = new Node<>(value);
            if (tail == null) {
                head = cur;
                tail = cur;
            } else {
                cur.last = tail;
                tail.next = cur;
                tail = cur;
                }
        }
        
        public T popFormHead() {
            if (head == null) {
                return null;
            }
            Node<T> cur = head;
            if (head == tail) {
                head = null;
                tail = null;
            } else {
                head = head.next;
                cur.next = null;
                head.last = null;
            }
            return cur.value;
        }
        
        public T popFormTail() {
            if (tail == null) {
                return null;
            }
            Node<T> cur = tail;
            if (head == tail) {
                head = null;
                tail = null;
            } else {
                tail = tail.last;
                cur.last = null;
                tail.next = null;
            }
            return cur.value;
        }
    }