78. 子集

输入: nums = [1,2,3]
输出:
[
  [3],
  [1],
  [2],
  [1,2,3],
  [1,3],
  [2,3],
  [1,2],
  []
]

  public List<List<Integer>> subsets(int[] nums) {
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        List<Integer> tmp = new ArrayList<Integer>();
        dfs(result, tmp, nums, 0);
        return result;
    }

    private void dfs(List<List<Integer>> result, List<Integer> tmp, int[] nums, int index) {
        result.add(new ArrayList<Integer>(tmp));
        for (int i = index; i < nums.length; i++) {
            tmp.add(nums[i]);
            dfs(result, tmp, nums, i + 1);
            tmp.remove(tmp.size() - 1);
        }
    }