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],
  []
]

   typedef vector< int > vi;  
   typedef vector< vector<int> > vvi;  

   #define tr(c,it) for(decltype((c).begin()) (it) = (c).begin(); (it) != (c).end(); (it)++)  

   static bool vviCompare(vi a, vi b)  
   {  
     return (a.size()<b.size());  
   }  

   static bool viCompare(int a, int b)  
   {  
     return (a<b);  
   }  

   vector<vector<int> > subsets(vector<int> &S) {  
     vvi res;  
     int n = S.size();  
     int total = 1<<n;  
     for(int i = 0; i <total; i++)  
     {  
       vi v;  
       for (int j = 0; j<31; j++)  
       {  
         if(i & 1<<j)  
         {  
           v.push_back(S[j]);  
         }  
       }  
       res.push_back(v);  
     }  
     std::sort(res.begin(), res.end(), vviCompare);  
     tr(res, it)  
     {  
       std::sort((*it).begin(),(*it).end(), viCompare);  
     }  
     return res;  
   }