(define reverse2
  (lambda (lst)
    (letrec ([helper
	      (lambda (ls new)
		(if (null? ls)
		    new
		  (helper (cdr ls) (cons (car ls) new))))])
	    (helper lst '()))))

(define reverse2!
  (lambda (lst)
    (if (null? lst)
	'()
      (let loop ([lst lst]
		 [prev '()]
		 [next (cdr lst)])
	   (set-cdr! lst prev)
	   (if (null? next)
	       lst
	     (loop next lst (cdr next)))))))