1.     public List<List<Integer>> subsets(int[] nums) {
    2.         List<List<Integer>> ans = new ArrayList<>();
    3.         generate(0, nums, ans, new ArrayList<>());
    4.         return ans;
    5.     }
    7.     private void generate(int level, int[] nums, List<List<Integer>> ans, List<Integer> tempList) {
    8.         if (level == nums.length) {
    9.             // why not tempList? tempList local value
    10.             ans.add(new ArrayList<>(tempList));
    11.             return;
    12.         }
    13.         tempList.add(nums[level]);
    14.         generate(level + 1, nums, ans, tempList);
    15.         tempList.remove(tempList.size() - 1);
    16.         generate(level + 1, nums, ans, tempList);
    17.     }