merge sort linked list

 Node* merge( Node* a , Node* b){
    Node* result = NULL;
    if (a == NULL)
        return (b);
    else if (b == NULL)
        return (a);

    if (a->data <= b->data) {
        result = a;
        result->next = Merge(a->next, b);
    }
    else {
        result = b;
        result->next = Merge(a, b->next);
    }
    return (result);
    }

   Node* sortList(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 merge(sortList(head) , sortList(headb));
        }