using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var sentence = "Hello I have letters and other-characters!";

        var old = new Dictionary<int, List<string>>(){{ 1, new List<string>(){ "a", "b", "c" }}};

        var output = new Dictionary<string, int>();

        old.Keys.Aggregate(output, (acc, oldKey) => {

                                                    foreach(var letter in old.GetValueOrDefault(oldKey))

                                                    {

                                                        acc.Add(letter, oldKey);

                                                    }

                                                                return acc;

                                                });

        Console.WriteLine(output["c"]);

    }

}