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.

def display_hierarchy(root_ad_unit, all_ad_units):
  """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.
  """
  # 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)