/in/foo.cc: In function 'int find(int, int)':
/in/foo.cc:7:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=pos[ch];i<a[ch].size();i++){
                    ~^~~~~~~~~~~~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:24:7: warning: array subscript has type 'char' [-Wchar-subscripts]
   a[ch].push_back(i);
       ^