LeetCode Problem : Subsets II

Problem

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

void swd_r(map<int,int>::const_iterator it,const map<int,int>& freq,int pos,
vector<int>& out,vector<vector<int> >& result)
{
    if(it == freq.end()){
        result.push_back(vector<int>(out.begin(),out.begin() + pos));
        return;
    }
    int num = it->first;
    int count = it->second;
    for(int j =count; j >= 0; --j){
        int cpos = pos;
        for(int i = 0; i < j; ++i){
            out[cpos] = num;
            ++cpos;
        }
        map<int,int>::const_iterator cit = it;
        swd_r(++cit,freq,cpos,out,result);
    }
}
vector<vector<int> > subsetsWithDup(vector<int> &S) {
    // IMPORTANT: Please reset any member data you declared, as
    // the same Solution instance will be reused for each test case.
    map<int,int> freq;
    int n = S.size();
    for(int i = 0;i< n;++i)
    ++freq[S[i]]; 
    vector<vector<int> > result;
    map<int,int>::const_iterator it = freq.begin();
    vector<int> out(n);
    swd_r(it,freq,0,out,result);
    return result;
}