二分乘法

二分乘法

 1 #define LL __int64
 2  
 3 //计算 a*b % mod
 4 LL Produc_Mod(LL a, LL b, LL mod) {
 5        LL sum = 0;
 6        while(b) {
 7               if(b & 1) sum = (sum + a) % mod;
 8               a = (a + a) % mod;
 9               b >>= 1;
10        }
11        return sum;
12 }
13  
14  
15 //计算a^b % mod
16 LL Power(LL a, LL b, LL mod) {
17        LL sum = 1;
18        while(b) {
19               if(b & 1) sum = Produc_Mod(sum, a, mod);
20               a = Produc_Mod(a, a, mod);
21               b >>= 1;
22        }
23        return sum;
24 }