公式: C(n,m) = n! / (m! * (n-m)!)
依据：
a/(b*c) = a * poww(b,mod,mod-2) % mod * poww(c,mod,mod-2) % mod;

代码块
求C(n,m)
const ll mod = 998244353;
ll jc[maxx];
void init() {
    jc[0] = 1;
    for (int i = 1; i < maxx; i++) jc[i] = i * jc[i - 1] % mod;
}
ll C(ll n, ll m) {
    return jc[n] * poww(jc[n - m], mod - 2, mod) % mod * poww(jc[m], mod - 2, mod) %mod;
}