foo.cc: In function 'int main()':
foo.cc:15:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<s.length();i++)
              ~^~~~~~~~~~~
foo.cc:36:18: warning: format '%s' expects argument of type 'char*', but argument 2 has type 'std::__cxx11::string* {aka std::__cxx11::basic_string<char>*}' [-Wformat=]
  printf("%s",&ans);
              ~~~~^

[Hydro](https://hydro.ac)提供评测服务