a function median in Scheme

Question

I am new to Scheme and I am using Dr.Racket to try to find the median of the list.

If the length of list L is odd, the function median returns the median element in the

Answer 1

This problem can be solved using the tortoise and hare algorithm, provided that a helper inner procedure is allowed - we need to pass two parameters for this to work. Other than that, all the restrictions are enforced:

(define (median lst)
  (define (median tortoise hare)
    (cond ((null? hare) 0)
          ((null? (cdr hare)) (car tortoise))
          (else (median (cdr tortoise) (cdr (cdr hare))))))
  (median lst lst))

It works as expected:

(median '(1))       ; returns 1
(median '(1 2))     ; returns 0
(median '(1 2 3))   ; returns 2
(median '(1 2 3 4)) ; returns 0