static int countRec ( int n , int sum ) {
  if ( n == 0 ) return sum == 0 ? 1 : 0 ;
  if ( lookup [ n ] [ sum ] != - 1 ) return lookup [ n ] [ sum ] ;
  int ans = 0 ;
  for ( int i = 0 ;
  i < 10 ;
  i ++ ) if ( sum - i >= 0 ) ans += countRec ( n - 1 , sum - i ) ;
  return lookup [ n ] [ sum ] = ans ;
}

static int countRec ( int n , int sum ) { if ( n == 0 ) return sum == 0 ? 1 : 0 ; if ( lookup [ n ] [ sum ] != - 1 ) return lookup [ n ] [ sum ] ; int ans = 0 ; for ( int i = 0 ; i < 10 ; i ++ ) if ( sum - i >= 0 ) ans += countRec ( n - 1 , sum - i ) ; return lookup [ n ] [ sum ] = ans ; }
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