Method addFirst

12_LinkedList.java/LL.java:14–23

(int data)

    public static int size;

    public void addFirst(int data) {
        size++;
        Node newNode = new Node(data);
        if(head == null) {
            head = tail = newNode;
        } else {
            newNode.next = head;
            head = newNode;
        }
    }

    public void addLast(int data) {
        size++;