Node* MergeLists(Node *headA, Node* headB)
{
    Node *res = NULL, **pp = &res, **tmp;

    while (headA && headB)
    {
        tmp = (headA->data < headB->data) ? &headA : &headB;
        *pp = *tmp;
        pp = &(*pp)->next;
        *tmp = (*tmp)->next;
    }
    *pp = (headA ? headA : headB);
    return res;
}