foo.cc: In function 'int main()':
foo.cc:4:32: error: 'endl' was not declared in this scope
     std::cout<<"Hello World!"<<endl;
                                ^~~~
foo.cc:4:32: 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。