/**
 * This one assumes that the list is ordered so that parents come before children
 */
public Tree listToTree(List<Item> list) {
  Map<Integer, Tree> refs = new HashMap<Integer, Tree>();
  
  Tree root = new Tree(0);
  Tree node = null
  for(Item item: list) {
    node = new Tree(item.id);
    refs.put(item.id, node)
    if(item.parent > 0) {
      refs.get(item.parent).children().add(node);
    } else {
      root.children().add(node);
    }
  }
  return root;
}

/**
 * This one does not assume any specific order of the list
 */
public Tree listToTree(List<Item> list) {
  Map<Integer, Tree> allNodes = new HashMap<Integer, Tree>();
  Map<Integer, Collection<Tree>> subNodes = new HashMap<Integer, Collection<Tree>>();

  Tree root = new Tree(0);
  Tree node = null
  for(Item item: list) {
    node = new Tree(item.id);
    allNodes.put(item.id, node);
    if(item.parent.id > 0) {
      if(subNodes.get(item.parent.id) == null) {
        subNodes.put(item.parent.id, new ArrayList<Tree>());
      }
      subNodes.get(item.parent.id).add(node);
    } else {
      root.children().add(node);
    }
  }

  for(Integer parent: subNodes.keySet()) {
    allNodes.get(parent).children(subNodes.get());
  }
  return root;

}