euler_3.cs

using System;
using System.Collections.Generic;
using System.Text;

namespace Largest_prime_factor
{
    class Program
    {
        static void Main(string[] args)
        {
            long target_number = 600851475143;

            long largest_factor = 0;

            while (target_number / iterator(target_number) != 1)
            {
                if (largest_factor < iterator(target_number)) largest_factor = iterator(target_number);
                target_number = target_number / iterator(target_number);
            }
            if (largest_factor < iterator(target_number)) largest_factor = iterator(target_number);

            Console.WriteLine(largest_factor);
            Console.Read();
        }

        static int iterator(long n)
        {
            for (int i = 2; i <= n; i++)
            {
                if (n % i == 0)
                    return i;
            }
            return 0;
        }

    }
}