ホーム - Project Euler

Problem 4¶

#!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)