Subsets------DFS

Given a set of distinct integers, return all possible subsets.

Have you met this question in a real interview? Yes
Example
If S = [1,2,3], a solution is:

[
[3],
[1],
[2],
[1,2,3],
[1,3],
[2,3],
[1,2],
[]
]
Note
Elements in a subset must be in non-descending order.
The solution set must not contain duplicate subsets.

public class Solution {
    public ArrayList<ArrayList<Integer>> subsets(int[] num) {
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        if(num == null || num.length == 0) {
            return result;
        }
        ArrayList<Integer> list = new ArrayList<Integer>();
        Arrays.sort(num);  
        subsetsHelper(result, list, num, 0);

        return result;
    }


    private void subsetsHelper(ArrayList<ArrayList<Integer>> result,
        ArrayList<Integer> list, int[] num, int pos) {

        result.add(new ArrayList<Integer>(list));

        for (int i = pos; i < num.length; i++) {

            list.add(num[i]);
            subsetsHelper(result, list, num, i + 1);
            list.remove(list.size() - 1);
        }
    }
}