This one was a simulation problem; simply keep checking the board
for any occurrence of a pattern where an apple would drop, and
iterate as long as any change is seen.

while (true) {
   bool changed = false ;
   for (int i=0; i+1<h; i++)
      for (int j=0; j<w; j++)
         if (b[i][j] == 'o' && b[i+1][j] == '.') {
            swap(b[i][j], b[i+1][j]) ;
            changed = true ;
         }
   if (!changed)
      break ;
}