快速乘

 1 #include<bits/stdc++.h>
 2 
 3 using namespace std;
 4 
 5 #define LL long long
 6 
 7 int main(){
 8     LL a,b,m;
 9     cin >> a >> b >> m;
10     LL ans = 0;
11     while(b){
12         if(b & 1) ans = (ans + a) % m;
13         a = (a<<1) % m;
14         b >>= 1;
15     }
16     cout << ans << endl;
17     return 0;
18 }