code: private static void displayInventoryTreeHelper(
        AdUnit root, Map<String, List<AdUnit>> treeMap, int depth) {
      System.out.println(generateTab(depth) + root.getName() + " (" + root.getId() + ")");
  
      if (treeMap.get(root.getId()) != null) {
        for (AdUnit child : treeMap.get(root.getId())) {
          displayInventoryTreeHelper(child, treeMap, depth + 1);
        }
      }
    }
docs: Helper for displaying inventory units.
  
  @param root the root inventory unit
  @param treeMap the map of id to {@code List} of inventory units
  @param depth the depth the tree has reached