Sherlock and Anagram - C# solution - SortedDictionary, Hashtable, BigInteger

SherlockAndAnagram13.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Collections;
using System.IO;
using System.Numerics;

static class Program
{
    static void Main(string[] args)
    {
        int t = Convert.ToInt32(Console.ReadLine());
        BigInteger[] results = new BigInteger[t];

        for (int i = 0; i < t; i++)
        {
            results[i] = UnorderedAnagrams(Console.ReadLine().Trim());
        }

        for (int i = 0; i < t; i++)
        {
            Console.WriteLine(results[i]);
        }
    }

    static BigInteger UnorderedAnagrams(string str)
    {
        Hashtable htPairs = new Hashtable();

        for (int len = 1; len <= str.Length; len++)
        {
            for (int i = 0; i + len <= str.Length; i++)
            {
                SortedDictionary<char, int> anagram = new SortedDictionary<char, int>();
                for (int j = i; j < i + len; j++)
                {
                    if (anagram.ContainsKey(str[j]))
                    {
                        anagram[str[j]] = (int)anagram[str[j]] + 1;
                    }
                    else
                    {
                        anagram.Add(str[j], 1);
                    }
                }

                string finalKey = "";
                foreach (char key in anagram.Keys)
                {
                    finalKey += key.ToString() + ((int)anagram[key]).ToString();
                }

                if (!htPairs.ContainsKey(finalKey))
                {
                    htPairs.Add(finalKey, 1);
                }
                else
                {
                    htPairs[finalKey] = (int)htPairs[finalKey] + 1;
                }
            }
        }

        BigInteger finalResult = 0;
        foreach (string k in htPairs.Keys)
        {
            finalResult += Combinatorial((int)htPairs[k], 2);
        }

        return finalResult;
    }

    public static BigInteger Combinatorial(long n, long r)
    {
        if (n < 0 || r < 0 || (n - r) < 0)
        {
            return 0;
        }

        return Factorial(n) / (Factorial(r) * Factorial(n - r));
    }

    public static BigInteger Factorial(long n)
    {
        if (n < 0)
        {
            return -1;
        }

        BigInteger result = 1;
        for (BigInteger i = 1; i <= n; i++)
        {
            result *= i;
        }

        return result;
    }
}