euler/Program.cs

using System;
using System.Collections.Generic;

namespace Euler {

  class Program {

  	static long Problem1() {
  		long sum = 0;
  		for (long i = 1; i < 1000; i++) {
  			if (i % 3 == 0 || i % 5 == 0) {
  				sum += i;
  			}
  		}
  		return sum;
  	}

  	static long Problem2() {
  		long max = 4_000_000;

  		var fibs = new List<long>();
  		fibs.Add(1);
  		fibs.Add(2);

  		while (fibs[fibs.Count - 1] < max) {
  			long num = fibs[fibs.Count - 1] + fibs[fibs.Count - 2];
 			  fibs.Add(num);
  		}

      long sum = 0;
  		foreach (int i in fibs) {
  			if (i % 2 == 0 && i <= max) {
  				sum += i;
  			}
  		}

  		return sum;
  	}

  	static bool IsPrime(long num, List<long> primes) {
  		foreach (long i in primes) {
  			if (num % i == 0) {
  				return false;
  			}
  		}
  		return true;
  	}

  	static List<long> PrimesUpThrough(long num) {
  		var primes = new List<long>();
  		primes.Add(2);
  		for (int i = 3; i <= num; i += 2) {
  			if (IsPrime(i, primes)) {
  				primes.Add(i);
  			}
  		}
  		return primes;
  	}

  	static long Problem3() {
  		long target = 600_851_475_143;
  		long targetSqrt = (long) Math.Sqrt(target);
  		List<long> primes = PrimesUpThrough(targetSqrt);
  		long highestPrimeFactor = 0;
  		foreach (long i in primes) {
  			if (target % i == 0) {
  				highestPrimeFactor = i;
  			}
  		}
  		return highestPrimeFactor;
  	}

    static void Main(string[] args) {
      Console.WriteLine(Problem3());
    }
  }
}