NCNU LISP HW3

exercise 3: linear recursive and linear iterative
                        but the function is changed:

            f(n) = n                        if n < 3 
                   2*f(n-1)+f(n-2)-f(n-3)   otherwise

(define (recursive x)
        (cond ((= x 0) 0)
              ((= x 1) 1)
              ((= x 2) 2)
              (else  (- (+ (* (recursive (- x 1)) 2) (recursive (- x 2)))
              (recursive (- x 3))))
        )
)
(define (interative x)
        (cond ((= x 0) 0)
              ((= x 1) 1)
              ((= x 2) 2)
              (else (do-interative 0 1 2 (- x 2)))
        )
)
(define (do-interative a b c x)
        (cond ((= x 0) c)
              (else (do-interative b c (- (+ (* c 2) b) a) (- x 1)))
        )
)