;; The first three lines of this file were inserted by DrScheme. They record metadata
;; about the language level of this file in a form that our tools can easily process.
#reader(lib "htdp-intermediate-lambda-reader.ss" "lang")((modname rev-fold) (read-case-sensitive #t) (teachpacks ()) (htdp-settings #(#t constructor repeating-decimal #f #t none #f ())))
;; A LON is either empty or (cons Number LON). 

;; LON -> LON 
;; reverse the order of the numbers on the list 

(check-expect (rev (list 1 2 3)) (list 3 2 1))

(define (rev l0) 
  (foldl (lambda (fst sofar) (cons fst sofar)) '() l0))

#| stress tests |#

(define (main n)
  (throw-away (time (rev (make-list n 0)))))
(define (throw-away x) 10)

(main 10)

;; SO FROM HERE:
;; 
;; List reverse(List l0) {
;;   for(l=l0, sofar = NULL; l!=NULL; l = l.rest) 
;;     sofar = cons(l.first,sofar); 
;; }
;; This is not an in-place update. But if the interviewer really asks you
;; for that, you can derive it in the same way.