快速乘（利用数的二进制进行计算）

#include<iostream>
using namespace std;
typedef long long ll;
ll q_mul(ll a, ll b, ll p) {
    ll ret = 0;
    while (b) {
        if (b&1)(ret += a)%=p;
        b>>=1;
        (a<<=1)%=p;
    }
    return ret;
}
int main() {
    ios::sync_with_stdio(0);
    ll a, b, p;
    cin >> a >> b >> p;
    cout << q_mul(a, b, p);
}