private static function displayInventoryTreeHelper(
        AdUnit $root,
        array $treeMap,
        $depth
    ) {
        $rootId = $root->getId();

        printf(
            "%s%s(%s)%s",
            self::generateTab($depth),
            $root->getName(),
            $rootId,
            PHP_EOL
        );

        if (!array_key_exists($rootId, $treeMap) || empty($treeMap[$rootId])) {
            return;
        }

        foreach ($treeMap[$rootId] as $child) {
            self::displayInventoryTreeHelper($child, $treeMap, $depth + 1);
        }
    }