/in/foo.cc:4:1: error: 'string' does not name a type
 string PalindromicString(string str){//目前还有BUG
 ^~~~~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:22:2: error: 'string' was not declared in this scope
  string str,res;
  ^~~~~~
/in/foo.cc:22:2: note: suggested alternatives:
In file included from /usr/include/c++/6/iosfwd:39:0,
                 from /usr/include/c++/6/ios:38,
                 from /usr/include/c++/6/ostream:38,
                 from /usr/include/c++/6/iostream:39,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/stringfwd.h:74:33: note:   'std::__cxx11::string'
   typedef basic_string<char>    string;
                                 ^~~~~~
/usr/include/c++/6/bits/stringfwd.h:74:33: note:   'std::__cxx11::string'
/in/foo.cc:23:2: error: 'cin' was not declared in this scope
  cin>>str;
  ^~~
/in/foo.cc:23:2: note: suggested alternative:
In file included from /in/foo.cc:1:0:
/usr/include/c++/6/iostream:60:18: note:   'std::cin'
   extern istream cin;  /// Linked to standard input
                  ^~~
/in/foo.cc:23:7: error: 'str' was not declared in this scope
  cin>>str;
       ^~~
/in/foo.cc:24:2: error: 'res' was not declared in this scope
  res=PalindromicString(str);
  ^~~
/in/foo.cc:24:27: error: 'PalindromicString' was not declared in this scope
  res=PalindromicString(str);
                           ^
/in/foo.cc:25:2: error: 'cout' was not declared in this scope
  cout<<res<<endl;
  ^~~~
/in/foo.cc:25:2: note: suggested alternative:
In file included from /in/foo.cc:1:0:
/usr/include/c++/6/iostream:61:18: note:   'std::cout'
   extern ostream cout;  /// Linked to standard output
                  ^~~~
/in/foo.cc:25:13: error: 'endl' was not declared in this scope
  cout<<res<<endl;
             ^~~~
/in/foo.cc:25:13: note: suggested alternative:
In file included from /usr/include/c++/6/iostream:39:0,
                 from /in/foo.cc:1:
/usr/include/c++/6/ostream:590:5: note:   'std::endl'
     endl(basic_ostream<_CharT, _Traits>& __os)
     ^~~~