Compilare: main.cpp: In function 'bool check(std::string)': main.cpp:9:23: warning: converting to non-pointer type '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'} from NULL [-Wconversion-null] 9 | x[x.size() - 1] = NULL; | ^~~~ main.cpp:14:22: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare] 14 | for(int j = 0; j < x.size(); j++){ | ~~^~~~~~~~~~ main.cpp:16:37: warning: converting to non-pointer type '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'} from NULL [-Wconversion-null] 16 | prel[prel.size() - 1] = NULL; | ^~~~ main.cpp:25:22: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare] 25 | for(int i = 0; i < prel.size(); i++){ | ~~^~~~~~~~~~~~~