list of prime factors

Question

I'm new to programming and Stack-overflow but I'm trying to create a list of prime factors from a number but it is returning more than one list and I don't know why. For example when I enter 10, it returns [2,5] and [2,5,5] instead of just [2,5]. Can someone explain what I'm doing wrong?

public class Solution {
    ArrayList<Integer> primelist = new ArrayList<>();

    ArrayList<Integer> findPrime(int num) {
        for (int i = 2; i <= num; i++) {
            if (num % i == 0) {
                primelist.add(i);
                num = num / i;
                if (num == 1) { break;}
                findPrime(num);
            }
        }
        System.out.println(primelist);
        return primelist;
    }

    public static void main(String[] args) {
        Solution sol = new Solution();
        sol.findPrime(30);//[2, 3, 5],[2, 3, 5, 5], [2, 3, 5, 5, 3, 5],[2, 3, 5, 5, 3, 5, 5]
        sol.findPrime(10);//[2, 5],[2, 5, 5]
    }
}

Answer 1

Im guessing you try to achieve the Sieve of Eratosthenes .

this works:

   @Test
    public void numbers() {

        class Solution {

            List<Integer> sieveOfEratosthenes(int n) {
                boolean prime[] = new boolean[n + 1];
                Arrays.fill(prime, true);
                for (int p = 2; p * p <= n; p++) {
                    if (prime[p]) {
                        for (int i = p * 2; i <= n; i += p) {
                            prime[i] = false;
                        }
                    }
                }
                List<Integer> primeNumbers = new LinkedList<>();
                for (int i = 2; i <= n; i++) {
                    if (prime[i]) {
                        primeNumbers.add(i);
                    }
                }
                return primeNumbers;
            }
        }
        //test it..
        Solution sol = new Solution();
        System.out.println(sol.sieveOfEratosthenes(10));
        System.out.println(sol.sieveOfEratosthenes(100));

    }

... which yields correct results:

[2, 3, 5, 7]
[2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97]

i borrowed this code from this article , an detailed explanation of the steps of the algorithm can be found there.