/in/foo.cc: In function 'int main()':
/in/foo.cc:12:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i = 0; i < s.size(); i++){
                 ~~^~~~~~~~~~
/in/foo.cc:19:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i = 0; i < s.size(); i++){
                 ~~^~~~~~~~~~
/in/foo.cc:30:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i = 0; i < s.size(); i++){
                 ~~^~~~~~~~~~