merge sort doubly linked list

Node* mergeSortedLists( Node* head1 , Node* head2){
   Node *result = NULL;
   if (head1 == NULL) {
      return head2;
   }
   if (head2 == NULL) {
      return head1;
   }
   if (head1->data < head2->data) {
      head1->next = mergeSortedLists(head1->next, head2);
      head1->next->prev = head1;
      head1->prev = NULL;
      return head1;
   } else {
      head2->next = mergeSortedLists(head1, head2->next);
      head2->next->prev = head2;
      head2->prev = NULL;
      return head2;
   }
}



struct Node *sortDoubly(struct Node *head)
{
       if(!head || !head->next) return head;
        Node* slow = head;
        Node* fast =head->next;
        while(fast && fast->next){
            fast= fast->next->next;
            slow = slow->next;
        }
        Node* headb = slow->next;
        slow->next = NULL;
        return mergeSortedLists(sortDoubly(head) , sortDoubly(headb));
}