Medium
应该可以秒的，但是i+1那里一开始写成了index+1

class Solution {
    public List<List<Integer>> subsets(int[] nums) {
        List<List<Integer>> res = new ArrayList<>();
        List<Integer> list = new ArrayList<>();
        dfsHelper(res, list, nums, 0);
        return res;    
    }

    private void dfsHelper(List<List<Integer>> res, List<Integer> list, int[] nums, int index){
        res.add(new ArrayList<Integer>(list));
        for(int i = index; i < nums.length; i++){
            if (list.contains(nums[i])){
                continue;
            }
            list.add(nums[i]);
            dfsHelper(res, list, nums, i + 1);
            list.remove(list.size() - 1);
        }
    }
}