using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public enum Ordinality

    {

        First,

        Last

    }

    public enum Cardinality

    {

        MostFrequent,

        LeastFrequent

    }

    public struct IndexFrequency

    {

        public int index;

        public int frequency;

    }

    static Dictionary<char, IndexFrequency> buildDictionary(string sentence)

    {

        Dictionary<char, IndexFrequency> dict = new Dictionary<char, IndexFrequency>();

        string lower = sentence.ToLower();

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

        {

            char myChar = lower[i];

            if (!dict.ContainsKey(myChar))

            {

                IndexFrequency myStruct;

                myStruct.index = i;

                myStruct.frequency = 1;

                dict.Add(myChar, myStruct);

            }

            else

            {

                IndexFrequency myStruct = dict[myChar];

                myStruct.frequency++;

                dict[myChar] = myStruct;

            }

        }

        List<KeyValuePair<char, IndexFrequency>> list = dict.ToList();

        //Console.WriteLine($"Dictionary: {string.Join('\n', list.Select(x => $"{x.Key}: {x.Value.index}, {x.Value.frequency}"))}");

        return dict;

    }

    static (char, int, int) repeatedCharacter(string input, Ordinality ordinality, Cardinality cardinality)

    {

        Dictionary<char, IndexFrequency> myDict = buildDictionary(input);

        char myChar = ' ';

        int myIndex = ordinality == Ordinality.First ? input.Length : -1;

        int myFrequency = cardinality == Cardinality.MostFrequent ? 0 : 1000;

        foreach (KeyValuePair<char, IndexFrequency> kvp in myDict)

        {

            if (cardinality == Cardinality.MostFrequent)

            {

                if (ordinality == Ordinality.First)

                {

                    if (kvp.Value.frequency > myFrequency)

                    {

                        myChar = kvp.Key;

                        myIndex = kvp.Value.index;

                        myFrequency = kvp.Value.frequency;

                    }

                }

                else

                {

                    if (kvp.Value.frequency >= myFrequency)

                    {

                        myChar = kvp.Key;

                        myIndex = kvp.Value.index;

                        myFrequency = kvp.Value.frequency;

                    }

                }

            }

            else

            {

                if (ordinality == Ordinality.First)

                {

                    if (kvp.Value.frequency < myFrequency)

                    {

                        myChar = kvp.Key;

                        myIndex = kvp.Value.index;

                        myFrequency = kvp.Value.frequency;

                    }

                }

                else

                {

                    if (kvp.Value.frequency <= myFrequency)

                    {

                        myChar = kvp.Key;

                        myIndex = kvp.Value.index;

                        myFrequency = kvp.Value.frequency;

                    }

                }

            }

        }   

        return (myChar, myIndex, myFrequency);

    }

    public static void Main()

    {

        string sentence = "ThisIsMySentence,ThereAreManyOthers,ButThisOneIsMine.";

        (char myChar, int myIndex, int myFrequency) = repeatedCharacter(sentence, Ordinality.First, Cardinality.MostFrequent);

        Console.WriteLine($"\nThe most frequent first character in this sentence: {sentence}\nis:{myChar} at index {myIndex} with frequency {myFrequency}");

        (myChar, myIndex, myFrequency) = repeatedCharacter(sentence, Ordinality.Last, Cardinality.MostFrequent);

        Console.WriteLine($"\nThe most frequent last character in this sentence: {sentence}\nis:{myChar} at index {myIndex} with frequency {myFrequency}");

        (myChar, myIndex, myFrequency) = repeatedCharacter(sentence, Ordinality.First, Cardinality.LeastFrequent);

        Console.WriteLine($"\nThe lest frequent first character in this sentence: {sentence}\nis:{myChar} at index {myIndex} with frequency {myFrequency}");

        (myChar, myIndex, myFrequency) = repeatedCharacter(sentence, Ordinality.Last, Cardinality.LeastFrequent);

        Console.WriteLine($"\nThe least frequent last character in this sentence: {sentence}\nis:{myChar} at index {myIndex} with frequency {myFrequency}");

    }

}