foo.cc: In function 'bool isVaild(std::string)':
foo.cc:14:18: 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 i=0;i<s.size();i++)
      |                 ~^~~~~~~~~
foo.cc: In function 'char f(char)':
foo.cc:9:1: warning: control reaches end of non-void function [-Wreturn-type]
    9 | }
      | ^

[Hydro](https://hydro.ac)提供评测服务