foo.cc:2:11: error: '::main' must return 'int'
 void main()
           ^
foo.cc: In function 'int main()':
foo.cc:8:1: error: 'cin' was not declared in this scope
 cin>>a[i];
 ^~~
foo.cc:8:1: note: suggested alternative:
In file included from foo.cc:1:0:
/usr/include/c++/7/iostream:60:18: note:   'std::cin'
   extern istream cin;  /// Linked to standard input
                  ^~~
foo.cc:10:1: error: 'cin' was not declared in this scope
 cin>>b[j];
 ^~~
foo.cc:10:1: note: suggested alternative:
In file included from foo.cc:1:0:
/usr/include/c++/7/iostream:60:18: note:   'std::cin'
   extern istream cin;  /// Linked to standard input
                  ^~~
foo.cc:26:1: error: 'cout' was not declared in this scope
 cout<<a[i]<<' '<<endl;
 ^~~~
foo.cc:26:1: note: suggested alternative:
In file included from foo.cc:1:0:
/usr/include/c++/7/iostream:61:18: note:   'std::cout'
   extern ostream cout;  /// Linked to standard output
                  ^~~~
foo.cc:26:18: error: 'endl' was not declared in this scope
 cout<<a[i]<<' '<<endl;
                  ^~~~
foo.cc:26:18: note: suggested alternative:
In file included from /usr/include/c++/7/iostream:39:0,
                 from foo.cc:1:
/usr/include/c++/7/ostream:590:5: note:   'std::endl'
     endl(basic_ostream<_CharT, _Traits>& __os)
     ^~~~
foo.cc:28:1: error: 'cout' was not declared in this scope
 cout<<b[j]<<' '<<endl;
 ^~~~
foo.cc:28:1: note: suggested alternative:
In file included from foo.cc:1:0:
/usr/include/c++/7/iostream:61:18: note:   'std::cout'
   extern ostream cout;  /// Linked to standard output
                  ^~~~
foo.cc:28:18: error: 'endl' was not declared in this scope
 cout<<b[j]<<' '<<endl;
                  ^~~~
foo.cc:28:18: note: suggested alternative:
In file included from /usr/include/c++/7/iostream:39:0,
                 from foo.cc:1:
/usr/include/c++/7/ostream:590:5: note:   'std::endl'
     endl(basic_ostream<_CharT, _Traits>& __os)
     ^~~~
foo.cc:30:1: error: 'cout' was not declared in this scope
 cout<<c[k]<<' '<<endl;
 ^~~~
foo.cc:30:1: note: suggested alternative:
In file included from foo.cc:1:0:
/usr/include/c++/7/iostream:61:18: note:   'std::cout'
   extern ostream cout;  /// Linked to standard output
                  ^~~~
foo.cc:30:18: error: 'endl' was not declared in this scope
 cout<<c[k]<<' '<<endl;
                  ^~~~
foo.cc:30:18: note: suggested alternative:
In file included from /usr/include/c++/7/iostream:39:0,
                 from foo.cc:1:
/usr/include/c++/7/ostream:590:5: note:   'std::endl'
     endl(basic_ostream<_CharT, _Traits>& __os)
     ^~~~

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