Subsets

class Solution {
    /**
     * @param S: A set of numbers.
     * @return: A list of lists. All valid subsets.
     */
    public ArrayList<ArrayList<Integer>> subsets(int[] nums) {
        // write your code here
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        if (nums == null || nums.length == 0){
            return result;
        }
        ArrayList<Integer> curList = new ArrayList<Integer>();
        Arrays.sort(nums);
        subsetsHelper(result, curList, nums, 0);
        return result;
        
    }
    
    private void subsetsHelper(ArrayList<ArrayList<Integer>> result, 
                          ArrayList<Integer> curList,
                          int[] nums,
                          int pos){
        result.add(new ArrayList<Integer>(curList));
        for(int i = pos; i < nums.length; i++){
            curList.add(nums[i]);
            subsetsHelper(result, curList, nums, i+1);
            curList.remove(curList.size() - 1);
        }
    }
}