A "Rackety" version of http://stackoverflow.com/a/33557880/343414

so.rkt

;; A "Rackety" version of http://stackoverflow.com/a/33557880/343414

#lang racket

(require rackunit)

(define (unwind xs)
  (let loop ([right '()] [turtle xs] [hare xs])
    (match* (hare turtle)
      [((list)         ts         ) (interleave right ts)]
      [((list x)       (cons t ts)) (cons t (interleave right ts))]
      [((list* _ _ hs) (cons t ts)) (loop (cons t right) ts hs)])))

(define (interleave xs ys)
  (match* (xs ys)
    [((list)      ys         ) ys]
    [(xs          (list)     ) xs]
    [((cons x xs) (cons y ys)) (cons x (cons y (interleave xs ys)))]))

(check-equal? (unwind '())
              '())

(check-equal? (unwind '(1 2 3))
              '(2 1 3))

(check-equal? (unwind '(1 1 2 3 5 8 13))
              '(3 2 5 1 8 1 13))

(check-equal? (unwind '(7 8 2 9 5 6))
              '(2 9 8 5 7 6))