求解带指数和阶乘的复杂数列求和问题 (模 20090327)

#include <iostream>
#include <algorithm>
#include <cmath>
#define MOD 20090327
using namespace std;

long long fac[1000005];
long long pow_mod(long long a, long long b) {
    long long res = 1;
    while (b) {
        if (b & 1) res = res * a % MOD;
        a = a * a % MOD;
        b >>= 1;
    }
    return res;
}

int main() {
    int n, k;
    cin >> n >> k;
    fac[0] = 1;
    for (int i = 1; i <= n; ++i) fac[i] = fac[i - 1] * i % MOD;
    long long ans = 0;
    long long tmp = 1;
    for (int i = 1; i <= n; ++i) {
        ans = (ans + fac[i] * pow_mod(k, 27 * (tmp - 1) / 2) % MOD) % MOD;
        tmp = tmp * 3 % MOD;
    }
    cout << ans << endl;
    return 0;
}