Euler3-Sieve.cs

using System;

class Program
{
    private const ulong N = 600851475143;
    
    static void Main()
    {
        bool[] composites = new bool[(ulong)Math.Sqrt(N)];
        ulong length = (ulong)composites.Length;
        ulong lpf = 1;

        // sieve of Eratosthenes algorithm
        for (ulong i = 2; (i * i) < length; i++)    
        {
            if (!composites[i])
            {
                for (ulong k = i; (i * k) < length; k++)
                {
                    composites[i*k] = true;
                }
            }
        }

        for (ulong i = length - 1; i > 1; i -= 2)
        {
            if (!composites[i] && (N % i) == 0)
            {
                lpf = i;
                break;
            }
        }

        Console.WriteLine(lpf);
    }
}