/in/foo.cc: In function 'int main()':
/in/foo.cc:15:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < s.length(); ++i) {
                     ~~^~~~~~~~~~~~
/in/foo.cc:24:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             for (int j = 0; j < a.length(); ++j) {
                             ~~^~~~~~~~~~~~
/in/foo.cc:38:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             for (int j = 0; j < a.length(); ++j) {
                             ~~^~~~~~~~~~~~
/in/foo.cc:48:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             for (int j = 0; j < b.length(); ++j) {
                             ~~^~~~~~~~~~~~
/in/foo.cc:62:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             for (int j = 0; j < a.length(); ++j) {
                             ~~^~~~~~~~~~~~
/in/foo.cc:72:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             for (int j = 0; j < b.length(); ++j) {
                             ~~^~~~~~~~~~~~
/in/foo.cc:14:9: warning: unused variable 'n' [-Wunused-variable]
     int n = s.length();
         ^