(define (itinerary flights)
  ;function to get the best matching pair according to string conditions
  (define (getBestPair start lst [pair '()])
    (if (empty? lst)
        pair
        ;if element we are expecting is a match
        (if (equal? start (car (car lst)))
            (cond
              ;case for getting first pair
              [(empty? pair) (getBestPair start (cdr lst) (car lst))]
              ;best pair condition
              [(string<? (cdr (car lst)) (cdr pair)) (getBestPair start (cdr lst) (car lst))]
              ;else skip pair
              [else (getBestPair start (cdr lst) pair)])
            ;skip pair
            (getBestPair start (cdr lst) pair))
        )
    )
  ;get list of all possible best pairs
  (define (getBestPairs start lst [res '()])
    (define pair (getBestPair start lst))
    (if (empty? pair)
        res
        (getBestPairs start (remove pair lst) (append res (list pair)))
        )
    )
  ;inspect all possible routes according to best pairs
  (define (getBestRoute start inspectList pairs [res '()])
    ;if no elements to inspect
    (if (empty? inspectList)
        ;add start because path gets appended before calling function for next pair to examine
        (append res (list start))
        (let ()
          ;if there are no pairs to inspect
          (if (empty? pairs)
              ;no such path
              #f
              (let ()
                (define pair (car pairs))
                (define newList (remove pair inspectList))
                (define newStart (cdr pair))
                ;get rest of the path (new arguments)
                (define newRes (getBestRoute newStart newList (getBestPairs newStart newList) (append res (list (car pair)))))
                ;catch the false state
                (if (eq? newRes #f)
                    ;revert old arguments and remove inspected pair
                    (getBestRoute start inspectList (remove pair pairs) res)
                    ;return result
                    newRes
                    )
                )
              )
          )
        )
    )

  (λ (start)
    (if (empty? (getBestPair start flights))
        ;if there is no best pair
        (error "No such itinerary!")
        (let ()
          (define pairs (getBestPairs start flights))
          (define route (getBestRoute start flights pairs))
          (if (eq? route #f)
              (error "No such itinerary!")
              route
              )
          )
        )
    )
  )
;((itinerary '(("A" . "C") ("C" . "A") ("A" . "B"))) "A")
(equal? ((itinerary '(("A" . "C") ("C" . "A") ("A" . "B"))) "A") '("A" "C" "A" "B"))
(equal? ((itinerary '(("G" . "E") ("E" . "F") ("A" . "G") ("D" . "E") ("B" . "C") ("E" . "C") ("A" . "B") ("C" . "D") ("F" . "A") ("C" . "A"))) "A")
        '("A" "B" "C" "A" "G" "E" "C" "D" "E" "F" "A"))
(equal? ((itinerary '(("SFO" . "HKO") ("YYZ" . "SFO") ("YUL" . "YYZ") ("HKO" . "ORD"))) "YUL") '("YUL" "YYZ" "SFO" "HKO" "ORD"))

;this line has to throw error
;((itinerary '(("SFO" . "COM") ("COM" . "YYZ"))) "COM")