Факториел + Фибоначи

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

namespace AlgorithmsOverStructureData
{
    class Program
    {
        static Dictionary<int> resultHistory = new Dictionary<int>();
        static void Main(string[] args)
        {
            Console.WriteLine(Factorial(5)); //5! = 5 * 4 * 3 * 2 * 1
            Console.WriteLine(FibonacciRecursiveOpt(6));

        }

        static long Factorial(int num)
        {
            if (num == 0)
            {
                return 1;
            }
            return num * Factorial(num - 1);
        }


        static int FibonacciRecursiveOpt(int n)
        {
            if (n == 0) return 0;
            if (n == 1) return 1;
            if (resultHistory.ContainsKey(n))
                return resultHistory[n];

            int result = FibonacciRecursiveOpt(n - 1) + FibonacciRecursiveOpt(n - 2);
            resultHistory[n] = result;

            return result;
        }

    }
}