static SinglyLinkedListNode mergeLists(SinglyLinkedListNode head1, SinglyLinkedListNode head2) {   
        if(head1==null){return head2;}
        if(head2==null){return head1;}
        
        SinglyLinkedListNode head3 = null;
        if(head1.data<head2.data){
            head3 = head1;
            head1=head1.next;
        }else{
            head3 = head2;
            head2 = head2.next;
        }
        SinglyLinkedListNode temp = head3;
        while(head1!=null&&head2!=null){
            if(head1.data<head2.data){
                temp.next = head1;
                head1 = head1.next;
            }else{
                temp.next = head2;
                head2 = head2.next;
            }
            temp = temp.next;
        }
        if(head1==null){
            temp.next = head2;
        }else{
            temp.next = head1;
        }
        return head3;
        
    }