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: 
Each combination's factors must be sorted ascending, for example: The factors of 2 and 6 is [2, 6], not [6, 2].
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]
]

    public List<List<Integer>> getFactors(int n) {
        List<List<Integer>> res = new ArrayList();
        if(n <= 1) return res;
        Set<List<Integer>> set = new HashSet();
        Helper(set, new ArrayList(), n);
        res.addAll(set);
        return res;
    }

    private void Helper(Set<List<Integer>> res, List<Integer> comb, int n){

        if(n == 1){
            res.add(comb);
        }else{
            int sqrt = (int)Math.sqrt(n);
            for(int i = 2; i <= sqrt ; i++){
                if(n % i == 0){
                    List <Integer> tmp = new ArrayList(comb);
                    tmp.add(i);
                    Helper(res, tmp, n / i);
                    tmp.add(n / i);
                    Collections.sort(tmp);
                    res.add(tmp);
                }
            }
        }
    }