A prime number is any number that can only be evenly divided by itself and 1. So for example 2, 3 and 5 are prime, but 6 and 9 are not.

Now, to find all the prime numbers that are factors in a number, we need to first fcheck if the number is even. If it is , then 2 is a factor, and then we keep dividing until the number is odd.

Then, starting at 3, and counting up in 2′s, we try and find a number which divides into the number, each time dividing by the divisor if a match is found.

 using System.Collections;

       public static int[] PrimeFactors(int num)
        {
            ArrayList factors = new ArrayList();

            bool alreadyCounted = false;
            while (num % 2 == 0)
            {
                if (alreadyCounted == false)
                {
                    factors.Add(2);
                    alreadyCounted = true;
                }
                num = num / 2;
            }

            int divisor = 3;
            alreadyCounted = false;
            while (divisor <= num)
            {
                if (num % divisor == 0)
                {
                    if (alreadyCounted == false)
                    {
                        factors.Add(divisor);
                        alreadyCounted = true;
                    }
                    num = num / divisor;
                }
                else
                {
                    alreadyCounted = false;
                    divisor += 2;
                }
            }

            int[] returnFactors = (int[])factors.ToArray(typeof(int));
     

            return returnFactors;
        }
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