Compilare: main.cpp: In function 'std::string sub(int&)': main.cpp:21:14: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare] 21 | while (i < s.length() && s[i] != ')' && s[i] != ']' && s[i] != '*') | ~~^~~~~~~~~~~~