public List<List<Integer>> subsets(int[] nums) {
List<List<Integer>> res = new ArrayList<>();
List<Integer> subset = new ArrayList<>();
getSubsets(nums, 0, subset, res);
public void getSubsets(int[] nums, int start, List<Integer> subset, List<List<Integer>> res) {
res.add(new ArrayList<>(subset));
for (int i = start; i < nums.length; i++) {
getSubsets(nums, i + 1, subset, res);
subset.remove(subset.size() - 1);