/in/foo.cc: In function 'int main()':
/in/foo.cc:7:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<a.length();i++){
              ~^~~~~~~~~~~
/in/foo.cc:21:35: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=a.length()-1,j=0;i>=0,j<a.length();i--,j++){
                                  ~^~~~~~~~~~~
/in/foo.cc:21:30: warning: left operand of comma operator has no effect [-Wunused-value]
  for(int i=a.length()-1,j=0;i>=0,j<a.length();i--,j++){
                             ~^~~
/in/foo.cc:24:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<a.length();i++){
              ~^~~~~~~~~~~