source: Common/NamedList.cs

using System.Collections.Generic;
using System.Linq;

namespace Common
{
    public class NamedList
    {
        public string Name;
        public List<string> List;
    }

    public class NamedLists : List<NamedList>
    {
        public void Add(string name, string listItemName)
        {
            NamedList item = this.FirstOrDefault(x => x.Name == name);
            if (item == null)
            {
                item = new NamedList { Name = name, List = new List<string>() };
                Add(item);
            }

            if (!item.List.Contains(listItemName))
            {
                item.List.Add(listItemName);
            }
        }

        public NamedLists GetReverse()
        {
            NamedLists result = new NamedLists();
            foreach (NamedList item in this)
            {
                foreach (string item2 in item.List)
                {
                    result.Add(item2, item.Name);
                }
            }
            return result;
        }
    }
}