message: Update brace-expansion.cpp
diff: @@ -39,7 +39,7 @@ private:
  }
  vector<string> formWords(const vector<vector<string>>& options) {
  - vector<string> words;
  + set<string> words_set;
  int total = 1;
  for (const auto& opt : options) {
  total *= opt.size();
  @@ -52,8 +52,8 @@ private:
  curr /= options[j].size();
  }
  reverse(tmp.begin(), tmp.end());
  - words.emplace_back(accumulate(tmp.cbegin(), tmp.cend(), string()));
  + words_set.emplace(accumulate(tmp.cbegin(), tmp.cend(), string()));
  }
  - return words;
  + return vector<string>(words_set.cbegin(), words_set.cend());
  }
  };
  