EulerLib.cs

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

namespace Euler
{
    public static partial class EulerLib
    {
        public static void Problem1 ()
        {
            /*
            If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9.
                The sum of these multiples is 23.
            Find the sum of all the multiples of 3 or 5 below 1000.
            */
            Console.WriteLine("The sum of all multiples of 3 or 5 below 1000:");

            var results = new List<int>();

            for (int i = 0; i < 1000; i++)
            {
                if ((i % 3 == 0) && (i % 5 != 0))
                    results.Add(i);
                
                if ((i % 5 == 0) && (i % 3 != 0))
                    results.Add(i);
                
                if ((i % 3 == 0) && (i % 5 == 0))
                    results.Add(i);
            }

            Console.WriteLine(results.Sum());
        }

        public static void Problem2()
        {
            /*
            Each new term in the Fibonacci sequence is generated by adding the previous two terms. 
                By starting with 1 and 2, the first 10 terms will be:
            1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...
            By considering the terms in the Fibonacci sequence whose values do not exceed 
                four million, find the sum of the even-valued terms.
            */
            Console.WriteLine("The sum of the even-valued terms in the Fibonacci sequence whose values do not exceed four million:");

            int[] terms = new int[3];
            terms[0] = 1;
            terms[1] = 2;

            int termsSum = 2;

            while (terms[2] <= 4000000)
            {
                if (terms[2] % 2 == 0)
                    termsSum += terms[2];
                
                terms[2] = terms[0] + terms[1];
                terms[0] = terms[1];
                terms[1] = terms[2];
            }

            Console.WriteLine(termsSum);
        }

        public static void Problem3()
        {
            /*
            The prime factors of 13195 are 5, 7, 13 and 29.
            What is the largest prime factor of the number 600851475143 ?
            */

            Console.WriteLine("The largest prime factor of 600851475143");
        }
    }
}