public void atLast(int new_data)
{
    Node new_node=new Node(new_data);
    if(head==null)
    {
        head=new Node(new_data);
        return;
    }
    new_node.next=null;
    Node last=head;
    while(last.next!=null)
    {
        last=last.next;
    }
    last.next=new_node;
    return;
}