subsets (java)

subsets.java

public class Solution {
    /**
     * @param nums: A set of numbers
     * @return: A list of lists
     */
    public List<List<Integer>> subsets(int[] nums) {
        Arrays.sort(nums);
        return helper(nums, nums.length - 1);
    }

    public List<List<Integer>> helper(int[] nums, int index) {
        List<List<Integer>> res;
        if (index < 0) {
            res = new ArrayList<List<Integer>>();
            res.add(new ArrayList<Integer>());
            return res;
        }
        List<List<Integer>> subs = helper(nums, index - 1);
        res = new ArrayList<List<Integer>>(subs);
        for (List<Integer> sub: subs) {
            List<Integer> s = new ArrayList<Integer>(sub);
            s.add(nums[index]);
            res.add(s);
        }
        return res;
    }
}