Method addFirst

12_LinkedList.java/DoubleLL.java:18–29

(int data)

    //add
    public void addFirst(int data) {
        Node newNode = new Node(data);
        size++;
        if(head == null) {
            head = tail = newNode;
            return;
        }

        newNode.next = head;
        head.prev = newNode;
        head = newNode;
    }

    //print
    public void print() {