【快速判断素数，10^9】

看到了一个很神的模板，具体什么意思还不理解，先马了再研究~

 1 bool prime (long long num)
 2 {
 3     if (num == 2 || num == 3 || num == 5)
 4         return true;
 5     if (num % 2 == 0 || num % 3 == 0 || num % 5 == 0 || num == 1)
 6         return false;
 7
 8     long long c = 7;
 9     int maxc = (int)(sqrt (num));
10     while (c <= maxc)
11     {
12         if (num % c == 0)
13             return false;
14         c += 4;
15         if (num % c == 0)
16             return false;
17         c += 2;
18         if (num % c == 0)
19             return false;
20         c += 4;
21         if (num % c == 0)
22             return false;
23         c += 2;
24         if (num % c == 0)
25             return false;
26         c += 4;
27         if (num % c == 0)
28             return false;
29         c += 6;
30         if (num % c == 0)
31             return false;
32         c += 2;
33         if (num % c == 0)
34             return false;
35         c += 6;
36     }
37     return true;
38 }