Subsets: generates all subsets of the series & amp; in ascending order.

Subsets: generates all subsets of a series in ascending order.

(1) assuming that the elements of the series are not repeated, for example, [4, 2, 3], we can see that there are a total of 2 ^ 3 = 8 subsets, and an empty set is added;

(2) sort the series to facilitate the generation of sub-sets in ascending order [2, 3, 4];

(3) Each subset corresponds to a binary value. 0-empty set 1-> (2) 2-> (3) 3-> (2, 3 )...... 7-> (2, 3, 4). The position of binary number 1 corresponds to the number of corresponding positions in the series. You can obtain the result by traversing it.

(4) Further, what should I do if there are repeated columns? For example, [2, 3, 3] seems complicated because the above method generates many identical subsequences. Yes, the same sub-sequence, but STL provides a good way to solve this problem, that is, the unique algorithm.

(5) unique algorithms should be used properly. Sort before use, and then resize. As follows:

vector
 
   > subsetsWithDup(vector
  
    &S) {       int n= S.size(),idx,tmp;       vector
   
     tuple;       vector
    
     > res;       res.push_back(tuple);       sort(S.begin(),S.end());       if(!n) return res;       for(int i=1; i<(1<
     
      > 1);           }           res.push_back(tuple);       }       sort(res.begin(),res.end());       auto it=unique(res.begin(),res.end());       res.resize(distance(res.begin(),it));       return res;    }