Educational Codeforces Round 83 (Rated for Div. 2) D. Count the Arrays

D. Count the Arrays

这个大佬说的挺好 来自

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const int maxn=4e5+7;
const int Mod=998244353;
ll fac[maxn];
ll ifac[maxn];
inline ll quickpow(ll a,ll n){
	 ll ans=1;
	for(;n;n>>=1){
		if(n&1) ans=(long long)ans*a%Mod;
		a=(long long)a*a%Mod;
	}
	return ans;
}
inline ll C(ll x,ll y){
	if(x<y) return 0;
	else return fac[x]*ifac[y]%Mod*ifac[x-y]%Mod;
}

ll ans=0;
int main(){
	int n,m;
	cin>>n>>m;
	if(n==2){
		cout<<0<<endl;
		return 0;
	}
	fac[0]=1;
	for(int i=1;i<=n+m;i++)	fac[i]=fac[i-1]*(ll)i%Mod;
	ifac[n+m]=quickpow(fac[n+m],Mod-2);
	for(int i=n+m-1;~i;i--) ifac[i]=ifac[i+1]*(ll)(i+1)%Mod;
	ans=C(m,n-1)*(n-2)%Mod;
	ans=ans*quickpow(2,n-3)%Mod;
	cout<<ans<<endl;
	return 0;
}