Subsets II

Given a collection of integers that might contain duplicates, 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 = [1,2,2], a solution is:

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

Code:

class Solution {
public:
    void findSub(vector<int> &S,int level,vector<int> &buf,vector<vector<int>> &res){
        for(int i=level;i<S.size();i++){
            buf.push_back(S[i]);
            res.push_back(buf);
            if(i<S.size()-1)
                findSub(S,i+1,buf,res);
            buf.pop_back();
            while(i<S.size()-1&&S[i]==S[i+1])
                i++;
        }
    }
    vector<vector<int> > subsetsWithDup(vector<int> &S) {
        vector<vector<int>> res;
        vector<int> buf;
        res.push_back(buf);
        sort(S.begin(),S.end());
        findSub(S,0,buf,res);
        return res;
    }
};