foo.cc: In function 'int main()':
foo.cc:6:2: error: 'cin' was not declared in this scope
  cin.getline(a,100);
  ^~~
foo.cc:6:2: note: suggested alternative: 'main'
  cin.getline(a,100);
  ^~~
  main
foo.cc:7:18: error: 'strlen' was not declared in this scope
  for (int i=0;i<=strlen(a);i++)
                  ^~~~~~
foo.cc:7:18: note: suggested alternative: 'struct'
  for (int i=0;i<=strlen(a);i++)
                  ^~~~~~
                  struct
foo.cc:14:2: error: 'cout' was not declared in this scope
  cout<<b;
  ^~~~

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