/in/foo.cc: In function 'int main()':
/in/foo.cc:8:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < s1.length(); i++)
                     ~~^~~~~~~~~~~~~
/in/foo.cc:12:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < s2.length(); i++)
                     ~~^~~~~~~~~~~~~
/in/foo.cc:55:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < ans.size(); i++)
                     ~~^~~~~~~~~~~~