组合数取mod

ll fact[maxn],inv[maxn];
ll Pow(ll x,ll n){
    ll ans=1,base=x;
    while(n){
        if(n&1) ans=ans*base%mod;
        base=base*base%mod;
        n>>=1;
    }
    return ans;
}
void init(){
    fact[0]=1;
    for (int i = 1; i < maxn; ++i)
    {
        fact[i]=fact[i-1]*i%mod;
    }
    inv[maxn-1]=Pow(fact[maxn-1],mod-2);
    for (int i = maxn-2; i >= 0; --i)
    {
        inv[i]=inv[i+1]*(i+1)%mod;
    }
}
ll C(ll n, ll m)
{
    if(n==m||m==0)
        return 1;
    if(m>n) return 0;
    return ((long long)fact[n]*inv[m]%mod)*inv[n-m]%mod;
}