乘法取模

 1 typedef long long LL;
 2 
 3 LL multi(LL a, LL b, LL mo){
 4     LL ans = 0;
 5     while(b){
 6         if(b & 1){
 7             ans += a;
 8             if(ans >= mo){
 9                 ans -= mo;
10             }
11         }
12         a <<= 1;
13         if(a >= mo){
14             a -= mo;
15             if(a == 0) return ans;
16         }
17         b >>= 1;
18     }
19     return ans;
20 }