foo.cc: In function 'void insert(char*)':
foo.cc:15:10: error: 'strlen' was not declared in this scope
  int len=strlen(s);
          ^~~~~~
foo.cc:15:10: note: suggested alternative: 'mbrlen'
  int len=strlen(s);
          ^~~~~~
          mbrlen

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