protected int[] getNodesIndices(@Nullable Collection<? extends TreeNode> children) {
    if (children == null) return ArrayUtilRt.EMPTY_INT_ARRAY;

    final int[] ints = new int[children.size()];
    int i = 0;
    for (TreeNode node : children) {
      ints[i++] = getIndex(node);
    }
    Arrays.sort(ints);
    return ints;
  }
protected int[] getNodesIndices(@Nullable Collection<? extends TreeNode> children)