ホーム - Project Euler

Problem 3¶

Largest Prime Factor¶

The prime factors of $13195$ are $5, 7, 13$ and $29$.
What is the largest prime factor of the number $600851475143$?

最大の素因数¶

13195 の素因数は 5, 7, 13, 29 である.
600851475143 の素因数のうち最大のものを求めよ.

#!r6rs
#!chezscheme

(import (chezscheme))

(define answer-3
  (let* ([n 600851475143]
         [check-max (isqrt n)])
    (let loop ([current 3] [rest n] [max-prime 1])
      (cond
        [(< check-max current) max-prime]
        [(zero? (mod rest current))
         (loop current (div rest current) current)]
        [else (loop (+ current 2) rest max-prime)]))))

(printf "3: ~D~%" answer-3)