View Compilation Error

0_0_35857442_4868.cpp: In static member function 'static std::string BigInteger::toString(int)':
0_0_35857442_4868.cpp:27:33: error: 'reverse' was not declared in this scope
   reverse(ret.begin(), ret.end());
                                 ^
0_0_35857442_4868.cpp: In static member function 'static std::string BigInteger::multi(std::string, std::string)':
0_0_35857442_4868.cpp:93:29: error: 'reverse' was not declared in this scope
   reverse(A.begin(), A.end());
                             ^