Leetcode: Subset II in C++

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],
  []
]

Solution:
vector<vector<int> > subsetsWithDup(vector<int> &S) {
       sort(S.begin(),S.end());
       vector<vector<int> > result(1);
       vector<int> tmp;
       int pre = S[0];
       //our working range. if there are duplicated element. Program will only apply to last working range. which will generate non duplicated subset.
       int range = 0; 
       int k = 0;
       int p = 0;
       for(int i=0;i<S.size();i++)
       {
           k = result.size()-1;
           if(i>0&&S[i]==pre)
           p = result.size()-range;
           else
           {
              p = 0;
              range = k+1;
           }
           while(p<=k)
           {
              tmp = result[p];
              tmp.push_back(S[i]);
              result.push_back(tmp);
              p++;
           }
           pre = S[i];
       }
       return result;
    }