Euler problem number #4

Question

Using Python, I am trying to solve problem #4 of the Project Euler problems. Can someone please tell me what I am doing incorrectly? The problem is to Find the larg

Answer 1

The question states:

What is the largest prime factor of the number 600851475143?

I solved this using C#, but the algorithm itself is language-agnostic.

1. Create a method for determining if a number is prime or not. This can be brute-force (rather than using a much more efficient sieving algorithm) and looks like this:

---

private static long IsPrime(long input)
        {
            if ((input % 2) == 0)
            {
                return 2;
            }
            else if ((input == 1))
            {
                return 1;
            }
            else
            {
                long threshold = (Convert.ToInt64(Math.Sqrt(input)));
                long tryDivide = 3;
                while (tryDivide < threshold)
                {
                    if ((input % tryDivide) == 0)
                    {
                        Console.WriteLine("Found a factor: " + tryDivide);
                        return tryDivide;
                    }
                    tryDivide += 2;
                }
                Console.WriteLine("Found a factor: " + input);
                return -1;
            }
        }

2. Once I have a function to determine primality, I can use this function to find the highest prime factor

---

private static long HighestPrimeFactor(long input)
{
    bool searching = true;
    long highestFactor = 0;
    while (searching)
    {
        long factor = IsPrime(input);
        if (factor != -1)
        {
            theFactors.Add(factor);
            input = input / factor; 
        }
        if (factor == -1)
        {
            theFactors.Add(input);
            highestFactor = theFactors.Max();
            searching = false;
        }
    }
    return highestFactor;
}

I hope this helps without giving too much away.