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