Compilare: user.cpp: In function ‘std::__cxx11::string* combine(const string&, const string&)’: user.cpp:11:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < str0.size(); ++ i) ~~^~~~~~~~~~~~~ user.cpp: In function ‘int solve(const string&, int)’: user.cpp:32:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if (border <= str.size() - border - 1) { ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~ user.cpp:35:24: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if (border != str.size() - border - 1 && str[str.size() - border - 1] == '?') ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~