/in/foo.cc: In function 'int main()':
/in/foo.cc:71:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for(int j=0;j<str.size();j++)
                     ~^~~~~~~~~~~
/in/foo.cc: In function 'bool topsort()':
/in/foo.cc:32:17: warning: iteration 26 invokes undefined behavior [-Waggressive-loop-optimizations]
         if(vis[i])
            ~~~~~^
/in/foo.cc:31:18: note: within this loop
     for(int i=1;i<=27;++i)
                 ~^~~~