/in/foo.cc: In function 'int main()':
/in/foo.cc:27:9: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  while(i<a.size()&&j<b.size()){
        ~^~~~~~~~~
/in/foo.cc:27:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  while(i<a.size()&&j<b.size()){
                    ~^~~~~~~~~
/in/foo.cc:36:9: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  while(i<a.size()){
        ~^~~~~~~~~
/in/foo.cc:40:9: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  while(j<b.size()){
        ~^~~~~~~~~
/in/foo.cc:44:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<c.size();i++){
              ~^~~~~~~~~