Then ..this is simpler version:-

 SinglyLinkedListNode*temp;
if(head1==NULL && head2==NULL){
    return NULL;
}
else if(head1!=NULL && head2==NULL){
    return head1;
}
else if(head1==NULL && head2!=NULL){
    return head2;
}

if(head1->data<=head2->data){
    temp=head1;
    temp->next= mergeLists(head1->next,head2);
}
else{
    temp=head2;
    temp->next= mergeLists(head1,head2->next);
}

return temp;