Submission #18237689

---

Source Code Expand

/*
after dusk passed,
there is a starry sky.
*/
#include <bits/stdc++.h>
#define inf 0x3f3f3f3f
#define m_k make_pair
#define mod 998244353
#define int long long
using namespace std;
const int N=5*1e5+100;
int n,k,fac[N],inv[N],ans;
inline void add(int &a,int b){a=((a+b>mod)?a+b-mod:a+b);}
inline void del(int &a,int b){a=((a-b<0)?a-b+mod:a-b);}
inline void mul(int &a,int b){a=(a*b)%mod;}
inline int C(int n,int m){return fac[n]*inv[n-m]%mod*inv[m]%mod;}
inline int m_pow(int a,int b)
{
	int ans=1;
	while (b)
	{
		if (b&1) mul(ans,a);
		b>>=1;
		mul(a,a);
	}
	return ans;
}
inline int read()
{
	int f=1,x=0;char s=getchar();
	while(s<'0'||s>'9'){if(s=='-')f=-1;s=getchar();}
	while(s>='0'&&s<='9'){x=x*10+s-'0';s=getchar();}
	return x*f;
}
inline int f(int n){return(C(n,(n+1)/2)-C(n,(n-3)/2)+mod)%mod;}
signed main()
{
	n=read();k=read();
	fac[0]=1;
	for (int i=1;i<=k;i++) fac[i]=fac[i-1]*i%mod;
	inv[k]=m_pow(fac[k],mod-2);
	for (int i=k-1;i>=0;i--) inv[i]=(inv[i+1]*(i+1))%mod;
	for (int i=n;i<=k;i++)
	{
		if (i<k-i) continue;
		if (i==k-i)
		{
			add(ans,(C(k-1,i)-C(k-1,2*i-n+1)+mod)%mod);
			continue;
		}
		if (2*i-n+1>k) add(ans,C(k,i));
		else add(ans,(C(k,i)-C(k,2*i-n+1)+mod)%mod);
	}
	printf("%lld\n",ans);
}

Submission Info

Judge Result