[Leetcode] Subsets

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],
  []
]

void helper(vector<int> S, vector<vector<int> >& ret, vector<int>& tmp, int k)
    {
        ret.push_back(tmp);
        for(int i=k; i<S.size(); i++)
        {
            tmp.push_back(S[i]);
            helper(S, ret, tmp, i+1);
            tmp.pop_back();
        }
    }

    vector<vector<int> > subsets(vector<int> &S) {
        sort(S.begin(), S.end());
        vector<vector<int> > ret;
        vector<int> tmp;
        helper(S, ret, tmp, 0);
        
        return ret;
    }