foo.cc:14:16: warning: multi-character character constant [-Wmultichar]
   14 |  if(c=='+'||c==' -'||c=='*'||c=='/')return true;
      |                ^~~~
foo.cc: In function 'void Match(std::string&)':
foo.cc:25:16: 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<s.length(); i++)
      |               ~^~~~~~~~~~~
foo.cc: In function 'bool fuhao(char)':
foo.cc:21:1: warning: control reaches end of non-void function [-Wreturn-type]
   21 | }
      | ^

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