使用HashSet的独特素数

Question

I wrote a method that recursively finds prime factors. Originally, the method simply printed values. I am currently trying to add them to a HashSet to find the unique prime factors.

In each of my original print statements, I added a primes.add() in order to add that particular integer into my set.

My printed output remains the same, for example, if I put in the integer 24 , I get 2*2*2*3 . However, as soon as I print the HashSet , the output is simply [2] .

    public static Set<Integer> primeFactors(int n)
    {
        Set<Integer> primes = new HashSet<Integer>();

        if(n <= 1)
        {
            System.out.print(n);
            primes.add(n);
        }
        else
        {
            for(int factor = 2; factor <= n; factor++)
            {
                if(n % factor == 0)
                {
                    System.out.print(factor);
                    primes.add(factor);
                    if(factor < n)
                    {
                        System.out.print('*');
                        primeFactors(n/factor);
                    }
                    return primes;
                }
            }
        }
        return primes;
    }

I have tried debugging via putting print statements around every line, but was unable to figure out why my .add() was not adding some values into my HashSet .

Answer 1

I seem to have fixed this by adding:

primes.addAll(primeFactors(n/factor));

Within my if-statement.

Answer 2

You could also reduce n. A bit faster.

        for (int factor = 2; factor <= n; factor++)
        {
            if (n % factor == 0)
            {
                System.out.print(factor);
                primes.add(factor);
                while (n % factor == 0)
                {
                     n /= factor;
                     System.out.print('*');
                     primeFactors(n/factor);
                }
            }
        }

If Set {2, 3} needs to be a List [2, 2, 2, 3] the add needs to be done in the while .