foo.cc: In function 'int main()':
foo.cc:21:15: warning: operation on 'i' may be undefined [-Wsequence-point]
   cout<<out2[i++]<<out2[i]<<" ";
              ~^~
foo.cc:14:7: warning: ignoring return value of 'char* fgets(char*, int, FILE*)', declared with attribute warn_unused_result [-Wunused-result]
  fgets(a, 100000, stdin);
  ~~~~~^~~~~~~~~~~~~~~~~~

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