Compilare: user.cpp: In function ‘ull convert_to_int(std::string)’: user.cpp:18:32: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < s.size(); ++i) { ^ user.cpp: In function ‘int main()’: user.cpp:66:47: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int len = 1; len <= now.size(); ++len) ^ user.cpp:67:51: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int j = 0; j <= now.size() - len; ++j) { ^