private void collectGced(@Nullable IntervalNode<T> root, @NotNull List<? super IntervalNode<T>> gced) {
    if (root == null) return;
    if (!root.hasAliveKey(true)) {
      gced.add(root);
    }
    collectGced(root.getLeft(), gced);
    collectGced(root.getRight(), gced);
  }
private void collectGced(@Nullable IntervalNode<T> root, @NotNull List<? super IntervalNode<T>> gced)