【快速判断素数，10^9】

 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 }