Q10235: Simply Emirp

/*
    注意：字串assign完之後要在最後一位後面加上 '\0'  不然會讀錯
    Run time: 0.040
*/
#include <stdio.h>
#include <string.h>
#define N 1000000
bool isprime[N+1];

int main()
{
    int i, j;

    for ( i = 2; i*i <= N; i++)
        if (!isprime[i])
            for ( j = 2*i; j <= N; j+=i)
                isprime[j] = true;

    int n;
    int reverse;
    int len;
    char str[7];
    char rev[7];

    while (scanf("%d", &n) != EOF)
    {
        if (isprime[n])
        {
            printf("%d is not prime.\n", n);
            continue;
        }

        snprintf(str, sizeof(str), "%d\0", n);

        len = strlen(str);

        for ( i = len-1, j = 0; i >= 0; i--)
            rev[j++] = str[i];

        rev[j] = '\0';

        sscanf(rev, "%d", &reverse);

        if (!isprime[reverse] && (n != reverse))
            printf("%d is emirp.\n", n);
        else
            printf("%d is prime.\n", n);

    }

    return 0;
}