foo.cc: In function 'int main()':
foo.cc:9:9: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)', declared with attribute warn_unused_result [-Wunused-result]
  freopen("yist.in","r",stdin);
  ~~~~~~~^~~~~~~~~~~~~~~~~~~~~
foo.cc:10:9: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)', declared with attribute warn_unused_result [-Wunused-result]
  freopen("yist.out","w",stdout);
  ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~

自豪的采用HydroJudge(https://github.com/hydro-dev/Hydro)进行评测。