/in/foo.cc: In function 'int main()':
/in/foo.cc:11:16: warning: converting to non-pointer type 'int' from NULL [-Wconversion-null]
   11 |         return NULL;
      |                ^~~~
/in/foo.cc:13:23: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   13 |     for (int i = 0; i < a.size(); i++) {
      |                     ~~^~~~~~~~~~