euler3

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

#include <stdio.h>

#define NUM    600851475143UL
/* #define NUM    13195 */

int isprime(int n)
{
    int i;

    for (i=3; i < n; i+=2) {
        if (!(n%i))
            return 0;
    }

    return 1;
}


int main(int argc, const char *argv[])
{
    unsigned long long i, prime, num;

    num = NUM;
    for (i=3; i <= num; i+=2) {
        if (!(NUM % i)) {
            printf("%llu\n", i);
            if (isprime(i)) {
                prime = i;
            }
            num /= i;
            printf("num: %llu\n", num);
        }
    }

    printf("Largest prime: %llu\n", prime);

    return 0;
}

is_prime 还可再优化。在下一道题中呈现。