View Compilation Error

0_0_30230522_12017.cpp: In function 'std::string add(std::string, std::string)':
0_0_30230522_12017.cpp:21:42: error: 'reverse' was not declared in this scope
     if(!flg) reverse(ans, ans + s1.size());
                                          ^
0_0_30230522_12017.cpp:22:42: error: 'reverse' was not declared in this scope
     else reverse(ans, ans + s1.size() + 1);
                                          ^