Combinatiosn.java

  public List<List<Integer>> subsets(int[] nums) {
    List<List<Integer>> result = new ArrayList<>();
    List<Integer> numList = new ArrayList<>();
    
    result.add(new ArrayList<>()); // empty set
    subsets(0, nums, numList, result);
    return result;
  }
  
  private void subsets(int offset, int[] nums, List<Integer> numList, List<List<Integer>> result) {
    if (offset >= nums.length) {
      return;
    }
    int val = nums[offset];
    // pick
    numList.add(val);
    subsets(offset + 1, nums, numList, result);
    // add to result
    result.add(new ArrayList<>(numList));
    // not pick
    numList.remove(numList.size() - 1);
    subsets(offset + 1, nums, numList, result);
  }