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#lang racket
; Definition for singly-linked list:
; val : integer?
; next : (or/c list-node? #f)
(struct list-node
(val next) #:mutable #:transparent)
; constructor
(define (make-list-node val [next-node #f])
(list-node val next-node))
(define/contract (add-two-numbers l1 l2)
(-> (or/c list-node? #f) (or/c list-node? #f) (or/c list-node? #f))
(define (process-list node [acc '()])
(if (list-node-next node)
(process-list (list-node-next node) (cons (list-node-val node) acc))
(cons (list-node-val node) acc)))
(define sum-of-lists (+ (string->number (apply ~a (process-list l1)))
(string->number (apply ~a (process-list l2)))))
(define sum-list-digits
(reverse
(map (λ (x) (string->number (string x)))
(string->list (number->string sum-of-lists)))))
(define (build-list l)
(if (empty? l)
#f
(make-list-node (car l) (build-list (cdr l)))))
(build-list sum-list-digits))
(define list1 (make-list-node 2 (make-list-node 4 (make-list-node 3))))
(define list2 (make-list-node 5 (make-list-node 6 (make-list-node 4))))
(add-two-numbers list1 list2)
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