Compilare: main.cpp: In function 'bool check(std::string)': main.cpp:9:18: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare] 9 | for(int i=0;i<s.size();i++) | ~^~~~~~~~~