@Html.RouteLink(routeName, null) //or similar overloads

public ActionResult Category(int? categoryId, CatalogPagingFilteringModel command){

var categoryIds = new List<int>();
if (category != null)
   {
       categoryIds.Add(category.Id);
   }
if (_catalogSettings.ShowProductsFromSubcategories)
   {
       //include subcategories
       categoryIds.AddRange(GetChildCategoryIds(category.Id));
   }