foo.cc: In function ‘bool huiwen(std::string)’:
foo.cc:7:15: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
    7 |         for(;i<s.size();i++)
      |              ~^~~~~~~~~
foo.cc:9:25: warning: logical not is only applied to the left hand side of comparison [-Wlogical-not-parentheses]
    9 |                 if(!s[i]==s[j])
      |                         ^~
foo.cc:9:20: note: add parentheses around left hand side expression to silence this warning
    9 |                 if(!s[i]==s[j])