using System;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        var words = new string[]{"vesal", "behrouzi", "test", "word"};

        var letters = new char[]{'a', 'b', 'c', 'd', 'e', 'f'};

        // Allocate 26 sets, one for each letter. These sets act as our indexing system.

        var places = new HashSet<Int32>[26];

        for (int i = 0; i < 26; i++)

        {

            places[i] = new HashSet<Int32>();

        }

​

        // Index the words: put index of words array into the relevant set

        for (int i = 0; i < words.Length; i++)

        {

            var word = words[i];

            for (int t = 0; t < word.Length; t++)

            {

                var location = word[t] - 'a';

                places[location].Add(i);

            }

        }

​

        // Lookup logic which is O(1)

        for (int i = 0; i < letters.Length; i++)

        {

            var location = letters[i] - 'a';

            Console.Write("Letter {0} ", letters[i]);

            if (places[location].Count > 0)

            {

                Console.WriteLine("can be found at {1} indexes", letters[i], String.Join(", ", places[location]));

            }

            else

            {

                Console.WriteLine("not found");

            }

        }

    }

}