单向链表

class Node {
    int val;
    Node next;
    Node(int val) {
        this.val = val;
    }
}

双向链表

class Node<E> {
        E item;
        Node<E> next;
        Node<E> prev;

        Node(Node<E> prev, E element, Node<E> next) {
            this.item = element;
            this.next = next;
            this.prev = prev;
        }
    }