/in/foo.cc: In function 'int main()':
/in/foo.cc:20:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < v2.size(); i++) {
                  ~~^~~~~~~~~~~
/in/foo.cc:23:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < v1.size(); i++) {
                  ~~^~~~~~~~~~~
/in/foo.cc:28:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < ans.size(); i++) {
                  ~~^~~~~~~~~~~~