is.rkt

#lang racket

(define (is ls)
  (let loop ([ls ls] [acc '()])
    (if (empty? ls)
        acc
        (loop (cdr ls) (ins acc (car ls))))))

(define (ins ls elem)
  (if (or (empty? ls) (< elem (car ls)))
      (cons elem ls)
      (cons (car ls) (ins (cdr ls) elem))))

(module+ test
  (require rackunit)
  (define tests
    '(((1 2 3 4) . (1 2 3 4))
      ((5 4 3 2) . (2 3 4 5))
      ((9 7 8 2) . (2 7 8 9))))
  (for ([t tests])
    (check-equal? (is (car t)) (cdr t))))