阶乘的逆元

int fac[maxn], facinv[maxn];
void getInv(int n) {
    fac[0] = 1;
    for (int i = 1; i <= n; ++ i) {
        fac[i] = (long long)fac[i - 1] * i % mod;
    }
    facinv[n] = fastPow(fac[n], mod - 2, mod);
    for (int i = n - 1; i > 0; -- i) {
        facinv[i] = (long long)facinv[i + 1] * (i + 1) % mod;
    }
}