/in/foo.cc: In function 'int main()':
/in/foo.cc:19:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0;i<a.size();i++)
                 ~^~~~~~~~~
/in/foo.cc:29:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0;i<a.size()/2;i++)
                 ~^~~~~~~~~~~
/in/foo.cc:15:9: warning: unused variable 'flag' [-Wunused-variable]
     int flag=1;
         ^~~~