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

Note:

• Elements in a subset must be in non-descending order.
• The solution set must not contain duplicate subsets.

For example,
If S = [1,2,3], a solution is:

[
  [3],
  [1],
  [2],
  [1,2,3],
  [1,3],
  [2,3],
  [1,2],
  []
]

public class Subsets {

	public List<List<Integer>> subsets(int[] S) {
		if (S == null)
			return null;
		Arrays.sort(S);
		List<List<Integer>> list = new ArrayList<List<Integer>>();
		list.add(new ArrayList<Integer>());
		for (int s : S)
			helper(list, s);
		return list;
	}

	public void helper(List<List<Integer>> lists, int val) {
		for (int i = 0; i < lists.size(); i++) {
			List<Integer> tmp = new ArrayList<Integer>(lists.get(i));
			tmp.add(val);
			lists.add(tmp);
		}
	}
}