Flatten tree view data

//Nested tree list, result list contains list of lists
 List<NavigationItem> allitems= result;

//Flatten the list
 List<NavigationItem> childitems = Flatten(allitems, a=>a.Items).ToList();


   public  List<NavigationItem> Flatten(List<NavigationItem> rootLevel, Func<NavigationItem, List<NavigationItem>> nextLevel)
        {
            List<NavigationItem> accumulation = new List<NavigationItem>();
            int i = 0;
            flattenLevel(accumulation, rootLevel, nextLevel, i);
            return accumulation;
        }

     
        private  void flattenLevel(List<NavigationItem> accumulation, List<NavigationItem> currentLevel, Func<NavigationItem, List<NavigationItem>> nextLevel, int level)
        {
            level++;

            foreach (NavigationItem item in currentLevel)
            {
             
                item.LevelNo = level;
                item.Level = "Level " + level.ToString();
                accumulation.Add(item);
                flattenLevel(accumulation, nextLevel(item), nextLevel, level);
            }
        }