Binary tree to list in post-order in racket

Question

Hi I am trying to transfer a binary tree to a list but crossing it in post-order.

(define (postorden arbol)
  (if (null? arbol) 
    ('()) (append (append (postorden (car (cdr arbol))) 
                          (postorden (car (cddr arbol)))) 
                  (list (car arbol)))))

But I get this error:

cdr: contract violation    expected: pair?    given: 'null

But for example with this tree: '((1 2) ((7 10) ((2 4) null null)) ((6 8) ((10 13) null null) null))

performed the cdr and get: (((7 10) ((2 4) null null)) ((6 8) ((10 13) null nul) null)) and then I make the car to this outcome: '((7 10) ((2 4) null null)) which if it is a pair. So I do not understand what is wrong.

Thank you in advance.

Answer 1

An if expression has the for (if e0 e1 e2) where e0,e1,and,e2 are expressions. In your program you have:

(if e0
    ('() ...)
    e2)

This means that if e0 is true, then ('() ...) is evaluated.

Here '() is the empty list also known as null.

The expression ('() ...) will attempt to call '() with the result of ..., and thus give an error.

Fix this error, and hopefully your other errors disappears - if not show us what you've and ask again.