/in/foo.cc: In function 'bool solve::dlx(int)':
/in/foo.cc:93:24: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
       for(int i = 0; i < new_sol.size(); i++)
                      ~~^~~~~~~~~~~~~~~~
/in/foo.cc: In function 'bool solve::construct_dancing_links()':
/in/foo.cc:205:24: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
       for(int j = 0; j < pos[i].size(); j++) {
                      ~~^~~~~~~~~~~~~~~
/in/foo.cc: In function 'void solve::solve()':
/in/foo.cc:251:26: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for(int t = 0; t < sol.size(); t++) {
                        ~~^~~~~~~~~~~~