code: def display_hierarchy(root_ad_unit, all_ad_units):
    
    # Create a dict mapping the ids of parents to lists of their children.
    parent_id_to_children = collections.defaultdict(list)
    for ad_unit in all_ad_units:
      if 'parentId' in ad_unit:
        parent_id_to_children[ad_unit['parentId']].append(ad_unit)
    parent_id_to_children = dict(parent_id_to_children)
  
    display_hierarchy_helper(root_ad_unit, parent_id_to_children, 0)
docs: Display the ad units as a tree.
  
    Args:
      root_ad_unit: The root ad unit to begin from.
      all_ad_units: A list containing all ad units.