void split(Link * L, Link * & A, Link * & B)
{ A=NULL; B=NULL;
  if (L==NULL) return;
  Link * lastA = L;  L = L->next;  lastA->next = NULL; A = lastA;
  if (L==NULL) return;
  Link * lastB = L;  L = L->next;  lastB->next = NULL; B = lastB;
  while (L != NULL)
  { lastA->next = L;  lastA = L;  L = L->next;  lastA->next = NULL;
    if (L == NULL) break;
    lastB->next = L;  lastB = L;  L = L->next;  lastB->next = NULL; } }