/in/foo.cc: In function 'int main()':
/in/foo.cc:14:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (i = 0; i < S.length(); i++) {
               ~~^~~~~~~~~~~~
/in/foo.cc:24:9: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   if (i == S.length())
       ~~^~~~~~~~~~~~~
/in/foo.cc:27:13: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    for (; i < S.length(); i++) {
           ~~^~~~~~~~~~~~
/in/foo.cc:33:10: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    if (i == S.length())
        ~~^~~~~~~~~~~~~