@Override
    public String toString() {
      StringBuilder sb = new StringBuilder("{");
      int[] offsets = keySet().toIntArray();
      Arrays.sort(offsets);
      for (int offset : offsets) {
        if (sb.length() > 1) sb.append(", ");
        sb.append(offset);
        Node node = get(offset);
        if (node != null) {
          sb.append(node);
        }
      }
      return sb.append("}").toString();
    }
@Override
    public String toString()