/in/foo.cc: In function 'int main()':
/in/foo.cc:24:16: warning: array subscript has type 'char' [-Wchar-subscripts]
     if(vis[b[0]]){
                ^
/in/foo.cc:27:13: warning: array subscript has type 'char' [-Wchar-subscripts]
     vis[b[0]]=1;
             ^