private function getSubPagesRecursive($rootPageId)
    {
        $pages = $this->getSubPages($rootPageId);
        foreach ($pages as $page) {
            if (false === $this->isPageTreeLeaf($page)) {
                ArrayUtility::mergeRecursiveWithOverrule(
                    $pages,
                    $this->getSubPagesRecursive($page['uid'])
                );
            }
        }

        return $pages;
    }