TEKPOOL

pseudo code:
assuming ascending.

[code:1]ptra=head of A;
prtb=head of B;
ptrc=new head;

if (ptra->val > ptrb->val) { ptrc = ptra; ptra= ptra->next; }
else { ptrc = ptrb; ptrb=ptrb->next; }

temp=ptrc
while (ptra && ptrb)
{
if (ptra->val > ptrb->val) { temp->next = ptra; ptra= ptra->next; }
else { temp->next=ptrb; ptrb=ptrb->next; }
}

//if any list remains, append
if (ptra) temp->next=ptra;
if (ptrb) temp->next=ptrb;
[/code:1]

that should do the job in O(N) and no extra space