Project Euler

[ホーム](https://redmine.noppi.jp) - [[Wiki|Project Euler]]

# [[Problem 4]]

```scheme

#!r6rs

#!chezscheme

(import (chezscheme))

(define (RotationNumber? num)

  (let* ([num-string (number->string num)]

         [str-length (string-length num-string)]

         [count (div str-length 2)])

    (let loop ([index 0])

      (cond

        [(<= count index) #t]

        [(equal?

           (string-ref num-string index)

           (string-ref num-string (- str-length 1 index)))

         (loop (add1 index))]

        [else #f]))))

(define answer-4

  (let loop1 ([num1 100] [result 0])

    (if (< 999 num1)

      result

      (let loop2 ([num2 100] [result result])

        (if (< 999 num2)

          (loop1 (add1 num1) result)

          (let ([num (* num1 num2)])

            (if (RotationNumber? num)

              (loop2 (add1 num2) (max result num))

              (loop2 (add1 num2) result))))))))

(printf "4: ~D~%" answer-4)

```