欧拉计划 第3题

The prime factors of 13195 are 5, 7, 13 and 29.

What is the largest prime factor of the number 600851475143 ?

 

class Program

    {

        private static List<long> primes = new List<long>(10000);



        static void Main(string[] args)

        {

            /*

             * The prime factors of 13195 are 5, 7, 13 and 29.

             * What is the largest prime factor of the number 600851475143 ?

             */

            const long number = 600851475143;

            primes.Add(2);

            long max = 2;

            long x = number;

            for (long i = 2; i <= x; i = NextPrime())

            {

			    if(number % i == 0)

			    {

			        x /= i;

			        max = i;

			    }

			}

            Console.WriteLine(max);



        }



        private static long NextPrime()

        {

            long last = primes.Last();

            long n = last + 1;

            while (true)

            {

                foreach (var item in primes)

                {

                    if (n % item == 0)

                    {

                        goto LoopOut;

                    }

                }

                primes.Add(n);

                return n;



                LoopOut:

                n++;

            }

        }

    }