public class TreeToHtmlConverter {
  public static String convert(TreeNode root) {
    if (root.isLeaf()) {
      return "leaf";
    }

    String childHtml = root.getChildren().stream()
        .map(child -> convert(child))
        .collect(Collectors.joining());

    return "<p>" + childHtml + "</p>";
  }
}

public class TreeToHtmlConverter {
  public static TreeToHtmlResult convert(TreeNode node) {
    List<String> specialTags = new ArrayList<>();
    String html = convert(node, specialTags);

    return new TreeToHtmlResult(html, specialTags);
  }

  public static String convert(TreeNode node, List<String> specialTags) {
    if (node.isLeaf()) {
      return "leaf";
    }

    if (node.isSpecial()) {
      specialTags.add(node.getName());
    }

    String childHtml = node.getChildren().stream()
        .map(child -> convert(child, specialTags))
        .collect(Collectors.joining());

    return "<p>" + childHtml + "</p>";
  }

  private static class TreeToHtmlResult {
    private final String html;
    private final List<String> specialTags;

    public TreeToHtmlResult(String html, List<String> specialTags) {
      this.html = html;
      this.specialTags = specialTags;
    }

    public String getHtml() {
      return this.html;
    }

    public List<String> getSpecialTags() {
      return this.specialTags;
    }
  }
}