Mapping a flat list to a hierarchical list with parent IDs C#

Question

I have a flat list of categories as shown in the following classes

public class FlatCategoryList
{
    public List Categories { get; set;         


        

Answer 1

Improved the suggested answer

public HieraricalCategoryList MapCategories(FlatCategoryList flatCategoryList)
{
    var categories = (from fc in flatCategoryList.Categories
                      select new Category() {
                          ID = fc.ID,
                          Name = fc.Name,
                          ParentID = fc.ParentID
                      }).ToList();

    var lookup = categories.ToLookup(c => c.ParentID);

    foreach(var c in rootCategories)//only loop through root categories
    {
        // you can skip the check if you want an empty list instead of null
        // when there is no children
        if(lookup.Contains(c.ID))
            c.ChildCategories = lookup[c.ID].ToList();
    }

    //if you want to return only root categories not all the flat list
    //with mapped child 

    categories.RemoveAll(c => c.ParentId != 0);//put what ever your parent id is

    return new HieraricalCategoryList() { Categories = categories };
}