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:29:37: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int border = 1; 2 * border <= str.size(); ++ border) { ~~~~~~~~~~~^~~~~~~~~~~~~ user.cpp:33:36: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = border; i < str.size() - border; ++ i) ~~^~~~~~~~~~~~~~~~~~~~~