題目要求：

Given a set of distinct integers, S, return all possible subsets.

Note:

• Elements in a subset must be in non-descending order.
• The solution set must not contain duplicate subsets.

For example,
If S = [1,2,3], a solution is:

[
  [3],
  [1],
  [2],
  [1,2,3],
  [1,3],
  [2,3],
  [1,2],
  []
]

class Solution {
public:
  vector<vector<int> > subsets(vector<int> &S) {
    vector<vector<int> > ans;
    vector<int> path;
    ans.push_back(path);
    sort(S.begin(), S.end());
    for(size_t i = 1; i <= S.size(); ++i)
    {
      path.clear();
      DFS(S, ans, path, 0, 0, i);
    }
    return ans;
  }
  
  void DFS(vector<int>& str, vector<vector<int> >& ans, vector<int>& path,
           int start, int count, int max_count)
  {
    if(count == max_count)
    {
      ans.push_back(path);
      return ;
    }
    for(size_t i = start; i < str.size(); ++i)
    {
      path.push_back(str[i]);
      DFS(str, ans, path, i + 1, count + 1, max_count);
      path.pop_back();
    }
  }
  
};