sclass LineComp_LinkedList {
  sclass Node {
    int value;
    Node prev, next;
  }

  Node tail;
  new MultiSetMap<Int, Node> indexByElement;
  
  *() {}
  *(L<Int> l) {
    fOr (int i : l) add(i);
  }
  
  void add(int i) {
    new Node n;
    n.value = i;
    n.prev = tail;
    if (tail != null) tail.next = n;
    tail = n;
    indexByElement.put(i, n);
  }
  
  void replacePair(int pa, int pb, int replaceWith, LineComp_PairCounts pairCounts) {
    Set<Node> setA = indexByElement.get(pa);
    if (setA != null) for (Node node : cloneList(setA)) {
      continue if node.value < 0;
      Node nb = node.next;
      if (nb != null && nb.value == pb) {
        if (node.prev != null) pairCounts.remove(node.prev.value, pa);
        pairCounts.remove(pa, pb);
        if (nb.next != null) pairCounts.remove(pb, nb.next.value);
        indexByElement.remove(pa, node);
        node.value = replaceWith;
        indexByElement.add(replaceWith, node);
        
        // delete nb
        indexByElement.remove(pb, nb);
        if (nb.next != null) nb.next.prev = node;
        node.next = nb.next;
        nb.value = -1; // mark invalid

        if (node.prev != null) pairCounts.add(node.prev.value, replaceWith);
        if (node.next != null) pairCounts.add(replaceWith, node.next.value);
      }
    }
  }
   
   L<Int> toList() {
    new L<Int> l;
    Node node = tail;
    while (node != null) {
      l.add(node.value);
      node = node.next;
    }
    ret reverseInPlace(l);
  }
}