foo.cc: In function ‘std::string jia(std::string)’:
foo.cc:7:33: warning: for increment expression has no effect [-Wunused-value]
    7 |         for(int i=a.size();i--;i>=0)
      |                                ~^~~
foo.cc: In function ‘std::string ch(std::string)’:
foo.cc:25:34: warning: for increment expression has no effect [-Wunused-value]
   25 |         for (int i=b.size();i--;i>=0)
      |                                 ~^~~