fibo

cr.cs

using System;
using System.Collections.Generic;

namespace ConsoleApp4
{
    internal class Program
    {
        private static void Main(string[] args)
        {
            Console.WriteLine(F(21));
            Console.WriteLine();

            foreach(var item in resultLookup)
            {
                Console.WriteLine($"{item.Key.ToString("00")} - {item.Value}");
            }

            Console.ReadKey();
        }

        private static Dictionary<int, int> resultLookup =
            new Dictionary<int, int>(
                new List<KeyValuePair<int, int>>() {
                    new KeyValuePair<int, int>(1, 1),
                    new KeyValuePair<int, int>(2, 2) });

        private static int F(int n)
        {
            if (n <= 0)
            {
                throw new ArgumentOutOfRangeException(nameof(n));
            }

            if (!resultLookup.ContainsKey(n))
            {
                int currentIndex = resultLookup.Count + 1;

                do
                {
                    resultLookup.Add(currentIndex, resultLookup[currentIndex - 1] + resultLookup[currentIndex - 2]);
                } while (++currentIndex <= n);
            }

            return resultLookup[n];
        }
    }
}