快速幂&快速乘

快速幂

#include <cstdio>
using namespace std;
typedef long long ll;

ll pow(ll a,ll b,ll p){
	ll ans = 1,t = a;
	while (b!=0){
		if (b & 1) ans *= t;
		ans %= p;
		t = (t*t) % p;
		b >>= 1;
	}
	return ans % p;
} 

int main(){
	ll a,b,p,ans;
	scanf("%lld%lld%lld",&a,&b,&p);
	printf("%lld\n",pow(a,b,p));
	return 0;
}

快速乘

#include <cstdio>
using namespace std;
typedef long long ll;

ll multi(ll a,ll b,ll p){
	ll ans = 0,t = 1;
	while (b!=0){
		if (b&1) ans += (t*a % p);
		ans %= p;
		t = t*2 % p;
		b >>= 1;
	}
	return ans;
} 

int main(){
	ll a,b,p,ans;
	scanf("%lld%lld%lld",&a,&b,&p);
	a %= p;
	b %= p;
	printf("%lld\n",multi(a,b,p));
	return 0;
}