/in/foo.cc: In function 'int main()':
/in/foo.cc:30:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < a1.size(); i++)
                    ^
/in/foo.cc:52:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < ans.size(); i++)
                    ^