Compilare: user.cpp: In function ‘int extend(std::string, int, int)’: user.cpp:14:36: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(sum = 0 ; j + sum < a.size() ; ++ sum) ^ user.cpp: In function ‘void buildz(std::string)’: user.cpp:22:32: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 1 ; i < a.size() ; ++ i) { ^ user.cpp: In function ‘int main()’: user.cpp:55:34: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = aux ; i < a.size() ; ++ i) ^