static SinglyLinkedListNode mergeLists(SinglyLinkedListNode head1, SinglyLinkedListNode head2) {
    SinglyLinkedListNode head = null;
    SinglyLinkedListNode tail = null;
        
    while( head1!=null || head2!=null) {
        SinglyLinkedListNode t = null ;
        if ( head2 == null || (head1!=null && head1.data <= head2.data) ) {
            t = head1;
            head1 = head1.next;
        } else {
            t = head2 ;
            head2 = head2.next;                
        }

        if (tail != null )
            tail.next = t;
        tail = t;
        if (head == null)
            head = tail;
    }
        
    tail.next = null;
    return head;
}