Hi, i have tried solving this problem with divide and conquer ( with a kind
of momoization with a c++ map ) with a recursive approach, but i cant get 
AC. I have tried with all the cases i have seen in this forum and i cant 
find the bug ( im getting WA ). Does anyone know what may be? may the 
recursive part be the problem? Has someone solved it in this way? Thanks. 
Eric.