78. Subsets

样例
输入
{1,8,5,4}

输出

[[], [1], [1, 4], [1, 4, 5], [1, 4, 5, 8], [1, 4, 8], [1, 5], [1, 5, 8], [1, 8], [4], [4, 5], [4, 5, 8], [4, 8], [5], [5, 8], [8]]

注意和46题不一样，递归传入的是i+1，递归路径如下，可以保证不会把重复元素放入结果中

helper(0) -> i=0,helper(1) -> i=1,helper(2) -> i=2,helper(3) -> i=3
                           -> i=2,helper(3) -> i=3
                           -> i=3
          -> i=1,helper(2) -> i=2,helper(3) -> i=3
          -> i=2,helper(3) -> i=3

 

public List<List<Integer>> permute(int[] nums) {
  List<List<Integer>> res = new ArrayList<>();
  List<Integer> temp = new ArrayList<>();
  helper(res, nums, 0, temp);
  return res;
}

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