254. Factor Combinations

Factor Combinations

Numbers can be regarded as product of its factors. For example,

8 = 2 x 2 x 2; = 2 x 4.

Write a function that takes an integer n and return all possible combinations of its factors.

Note:

You may assume that n is always positive. Factors should be greater than 1 and less than n.

Examples: input: 1 output:

[]

input: 37 output:

[]

input: 12 output:

[ [2, 6], [2, 2, 3], [3, 4] ]

input: 32 output:

[ [2, 16], [2, 2, 8], [2, 2, 2, 4], [2, 2, 2, 2, 2], [2, 4, 4], [4, 8] ]

Solution

public class Solution {
     public List<List<Integer>> getFactors(int n) {
        List<List<Integer>> result = new ArrayList<>();
        helper(result, new ArrayList<>(), n, 2);
        return result;
    }

    private void helper(List<List<Integer>> result, List<Integer> current, int n, int start) {
        if (n <= 1) {
            if (current.size()>=2) {
                result.add(new ArrayList<Integer>(current));
            }

            return;
        }

        for (int i = start; i <= n; i++) {
            if (n % i == 0) {
                current.add(i);
                helper(result, current, n / i, i);
                current.remove(current.size() - 1);
            }
        }
    }
}