/in/foo.cc: In function 'int main()':
/in/foo.cc:19:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<a.size();i++)
              ~^~~~~~~~~
/in/foo.cc:18:7: warning: unused variable 'flag' [-Wunused-variable]
  bool flag=0;
       ^~~~