AcWing 90. 64位整数乘法

求 a 乘 b 对 p 取模的值。
输入格式

第一行输入整数a
，第二行输入整数b，第三行输入整数p

。
输出格式

输出一个整数，表示a*b mod p的值。
数据范围

$1\le a,b,p\le 10^{18}$1≤a,b,p≤1018
输入样例：

3
4
5

输出样例：

2

模板

int fp(int a, int b) {
	int ret = 0;
	while(b) {
		if(b & 1) ret = ( a + ret ) % mod;
		b >>= 1;
		a = (a<<1) % mod;
	}
	return ret;
}

完整代码

#define debug
#ifdef debug
#include <time.h>
#include "/home/majiao/mb.h"
#endif

#include <iostream>
#include <algorithm>
#include <vector>
#include <string.h>
#include <map>
#include <set>
#include <stack>
#include <queue>
#include <math.h>

#define MAXN ((int)1e5+7)
#define ll long long int
#define INF (0x7f7f7f7f)
#define fori(lef, rig) for(int i=lef; i<=rig; i++)
#define forj(lef, rig) for(int j=lef; j<=rig; j++)
#define fork(lef, rig) for(int k=lef; k<=rig; k++)
#define QAQ (0)

using namespace std;

#define show(x...) \ do { \ cout << "\033[31;1m " << #x << " -> "; \ err(x); \ } while (0)

void err() { cout << "\033[39;0m" << endl; }
template<typename T, typename... A>
void err(T a, A... x) { cout << a << ' '; err(x...); }

namespace FastIO{

	char print_f[105];
	void read() {}
	void print() { putchar('\n'); }

	template <typename T, typename... T2>
		inline void read(T &x, T2 &... oth) {
			x = 0;
			char ch = getchar();
			ll f = 1;
			while (!isdigit(ch)) {
				if (ch == '-') f *= -1; 
				ch = getchar();
			}
			while (isdigit(ch)) {
				x = x * 10 + ch - 48;
				ch = getchar();
			}
			x *= f;
			read(oth...);
		}
	template <typename T, typename... T2>
		inline void print(T x, T2... oth) {
			ll p3=-1;
			if(x<0) putchar('-'), x=-x;
			do{
				print_f[++p3] = x%10 + 48;
			} while(x/=10);
			while(p3>=0) putchar(print_f[p3--]);
			putchar(' ');
			print(oth...);
		}
} // namespace FastIO
using FastIO::print;
using FastIO::read;

#define int unsigned long long
int n, m, Q, K, mod;

int fp(int a, int b) {
	int ret = 0;
	while(b) {
		if(b & 1) ret = ( a + ret ) % mod;
		b >>= 1;
		a = (a<<1) % mod;
	}
	return ret;
}

signed main() {
#ifdef debug
	freopen("test", "r", stdin);
	clock_t stime = clock();
#endif
	scanf("%llu %llu %llu ", &n, &m, &mod);
	int ans = fp(n, m);
	printf("%llu\n", ans);




#ifdef debug
	clock_t etime = clock();
	printf("rum time: %lf 秒\n",(double) (etime-stime)/CLOCKS_PER_SEC);
#endif 
	return 0;
}