P5104 红包发红包(概率+飞马小定理求分数逆元)

传送门

题目描述:

思路:

原博客传送门

AC代码:

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const int maxn = 10000005;
const int inf = 0x3f3f3f3f;
const ll mod = 1000000007;
ll qpow(ll a, ll n, ll p) {
    ll res = 1;
    while (n) {
        if (n & 1) {
            res = (res * a) % p;
        }
        a = (a * a) % p;
        n >>= 1;
    }
    return res;
}
int main() {
    ll w, n, k; scanf("%lld%lld%lld", &w, &n, &k);
    printf("%lld", w * qpow(qpow(2, k, mod), mod - 2, mod) % mod);
    return 0;
}