（Problem 37）Truncatable primes

The number 3797 has an interesting property. Being prime itself, it is possible to continuously remove digits from left to right, and remain prime at each stage: 3797, 797, 97, and 7. Similarly we can work from right to left: 3797, 379, 37, and 3.

Find the sum of the only eleven primes that are both truncatable from left to right and right to left.

NOTE: 2, 3, 5, and 7 are not considered to be truncatable primes.

题目大意：

3797这个数很有趣。它本身是质数，而且如果我们从左边不断地裁去数字，得到的仍然都是质数：3797,797,97,7。而且我们还可以从右向左裁剪：3797,379,37,3，得到的仍然都是质数。

找出全部11个这样从左向右和从右向左都可以裁剪的质数。
注意：2,3,5和7不被认为是可裁剪的质数。

//（Problem 37）Truncatable primes
// Completed on Thu, 31 Oct 2013, 13:12
// Language: C
//
// 版权所有（C）acutus   (mail: acutus@126.com) 
// 博客地址：http://www.cnblogs.com/acutus/
#include<stdio.h>
#include<math.h>
#include<string.h>
#include<ctype.h>
#include<stdlib.h>
#include<stdbool.h>

bool isprim(int n)
{
    int i=2;
    if(n==1) return false;
    for(; i*i<=n; i++)
    {
        if(n%i==0)  return false;
    }
    return true;
}

bool truncatable_prime(int n)
{
    int i,j,t,flag=1;
    char s[6];
    int sum=0;
    sprintf(s,"%d",n);
    int len=strlen(s);

    if(!isprim(s[0]-'0') || !isprim(s[len-1]-'0')) return false;

    for(i=1; i<len-1; i++)
    {
        t=s[i]-'0';
        if(t==0 || t==2 || t==4 || t==6 || t==5 || t==8)  return false;
    }
    
    for(i=1; i<len-1; i++)
    {
        for(j=i; j<len-1; j++)
        {
            sum+=s[j]-'0';
            sum*=10;
        }
        sum+=s[j]-'0';
        if(!isprim(sum))  return false;
        sum=0;
    }
    j=len-1;
    i=0;
    while(j>i)
    {
        for(i=0; i<j; i++)
        {
            sum+=s[i]-'0';
            sum*=10;
        }
        sum+=s[i]-'0';
        if(!isprim(sum)) return false;
        sum=0;
        i=0;
        j--;
    }
    return true;
}

int main()
{
    int sum,count;
    sum=count=0;
    int i=13;
    while(1)
    {
        if(isprim(i) && truncatable_prime(i))
        {
            count++;
            sum+=i;
            //printf("%d
",i);
        }
        i=i+2;
        if(count==11)  break;
    }
    printf("%d
",sum);
    return 0;
}

Answer:

748317