1 665c255d 2023-08-04 jrmu ;; weiqun zhang's solution
 2 665c255d 2023-08-04 jrmu 
 3 665c255d 2023-08-04 jrmu (define (make-queue)
 4 665c255d 2023-08-04 jrmu   (let ((front-ptr '())
 5 665c255d 2023-08-04 jrmu 	(rear-ptr '()))
 6 665c255d 2023-08-04 jrmu     (define (prt-q)
 7 665c255d 2023-08-04 jrmu       (newline)
 8 665c255d 2023-08-04 jrmu       (display front-ptr))
 9 665c255d 2023-08-04 jrmu     (define (emp-q?)
10 665c255d 2023-08-04 jrmu       (null? front-ptr))
11 665c255d 2023-08-04 jrmu     (define (fro-q)
12 665c255d 2023-08-04 jrmu       (if (emp-q?)
13 665c255d 2023-08-04 jrmu 	  (error "FRONT called with an empty queue")
14 665c255d 2023-08-04 jrmu 	  (car front-ptr)))
15 665c255d 2023-08-04 jrmu     (define (ins-q! item)
16 665c255d 2023-08-04 jrmu       (let ((new-pair (cons item '())))
17 665c255d 2023-08-04 jrmu 	(cond ((emp-q?)
18 665c255d 2023-08-04 jrmu 	       (set! front-ptr new-pair)
19 665c255d 2023-08-04 jrmu 	       (set! rear-ptr new-pair))
20 665c255d 2023-08-04 jrmu 	      (else
21 665c255d 2023-08-04 jrmu 	       (set-cdr! rear-ptr new-pair)
22 665c255d 2023-08-04 jrmu 	       (set! rear-ptr new-pair)))))