SinglyLinkedListNode* mergeLists(SinglyLinkedListNode* head1, SinglyLinkedListNode* head2) {

SinglyLinkedListNode* temp;

if(head1==NULL&&head2==NULL){
    return NULL;

}

if(head1==NULL) return head2;
if(head2==NULL) return head1;

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

else{
   head2->next=mergeLists(head2->next,head1);
    return head2;
}

} well..I think my code is easier to understand