#lang scheme
;; UMBC CMSC 331 Fall 2017, HW7, YOUR NAME, YOUR_USERNAME@UMBC.EDU

(define (pass) null)

;; (1) Enigma: put your answer to problem one as a comment here.  User
;; as many lines as you need

(define (enigma x)
  ;; the enigma function ...
  (or (null? x)
      (null? (rest x))
      (and (not (equal? (car x) (car (cdr x))))
           (enigma (rest x)))))

;; (2) connumdrum put your answer to problem two as a comment here.
;; User as many lines as you need.

(define (conundrum x y)
  ;; The amazing function conundrum ...
  (cond ((null? y) -1)
        ((equal? x (first y)) 0)
        (else (let ((z (conundrum x (rest y))))
                   (if (< z 0) z (+ 1 z))))))


;; (3.1) Replace ONLY the commented with your actual implementation,
;; W/O CHANGING THE FIRST LINE!

(define do-nothing
  null ;; replace this line with a LAMBDA EXPRESSION
  )

;; (3.2) Replace ONLY the commented with your actual implementation,
;; W/O CHANGING THE FIRST LINE!

(define first-str
  null ;; replace this line with a LAMBDA EXPRESSION
  )

;; (3.3) Replace ONLY the commented with your actual implementation,
;; W/O CHANGING THE FIRST LINE!

(define converse-func
  null ;; replace this line with a LAMBDA EXPRESSION
  )

;; complete the following as answers to the remaining problems.  add
;; as many additional functions as needed.  Please add appropriate
;; comments to your functions.

;; (4) unique-atoms

(define (unique-atoms s)
  ;; given an s-expression s, returns a list of the unique atoms in s.
  ;; The order is not important.  e.g.: (unique-atoms? '(a a)) => (a),
  ;; (unique-atoms? '(a (b (c (a))))) => (a b c), (unique-atoms? '())
  ;; => ()
  (pass))


;; (5) reverse-tree

(define (reverse-tree s)
  ;; given an s-expression s, returns the complete reversal of that
  ;; s-expression: not only the top-level list, but every nested sub-list,
  ;; is reversed.
  ;; So, "(reverse-tree '(a b (c d) (e (f g h))))" will yield:
  ;; '(((h g f) e) (d c) b a)
  (pass))

(provide
  enigma conundrum
  do-nothing first-str converse-func
  unique-atoms
  reverse-tree)