perl/p3.pl


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#!/usr/bin/env perl

use strict;
use warnings;

use List::Util qw(all);

my $n = 600851475143;

# Functional

# Imperative
my @primes;
my $last_factor;

for my $i (2 .. $n) {
    last if ($i > $n);
    if (all { $i % $_ != 0 } @primes) {
        if ($n % $i == 0) {
            $n /= $i;
            $last_factor = $i;
            redo;
        }
        push(@primes, $i);
    }
}
print "$last_factor\n";