public void addAtBegin(int item){
  Node nn = new Node();
  nn.data = item;
  
  //attach new node to the previous node 
  if(this.size >= 1)
    nn.next = head;
  else
    this.tail = nn;
  
  //shift head of previous node to the ned node
  this.head = nn;
  this.size++;
}