View Compilation Error

0_0_22104290_27335.cpp: In function 'std::string add(std::string, std::string)':
0_0_22104290_27335.cpp:9:27: error: 'reverse' was not declared in this scope
  reverse(a.begin(),a.end());
                           ^
0_0_22104290_27335.cpp: In function 'std::string mul(std::string, std::string)':
0_0_22104290_27335.cpp:29:27: error: 'reverse' was not declared in this scope
  reverse(a.begin(),a.end());
                           ^