sumBigNumber_v2.cpp

 std::string sum(const std::string& a, const std::string& b)
 {
   std::string res(std::max(a.size(), b.size()) + 1, '0');
   auto iterA = a.rbegin();
   auto iterB = b.rbegin();
   auto iter = res.rbegin();
   static const auto addIter = [&iter](decltype(iterA)& iterA, const std::string& a)
   {
     if(iterA != a.rend())
     {
       *iter += *iterA - '0';
       if(*iter > '9')
       {
         auto tmpIter = iter;
         ++tmpIter;
         *tmpIter = '1';
         *iter -= 10;
       }
       ++iterA;
     }
   };
   while(iter != res.rend())
   {
     addIter(iterA, a);
     addIter(iterB, b);
     ++iter;
   }
   if(res[0] == '0')
   {
     res.erase(0, 1);
   }
   return res;
 }