BZOJ4555: [Tjoi2016&Heoi2016]求和

把题目式子结合第二类斯特林数通项公式化一化就变成NTT模板题了：

\[f(n)=\sum_{i=0}^n\sum_{j=0}^iS(i,j)*2^j*j!$$ $$=\sum_{i=0}^n\sum_{j=0}^nS(i,j)*2^j*j!$$ $$=\sum_{i=0}^n\sum_{j=0}^n2^j\sum_{k=0}^j(-1)^k\binom{j}{k}(j-k)^i$$ $$=\sum_{i=0}^n\sum_{j=0}^n2^j\sum_{k=0}^j(-1)^k\frac{j!}{k!(j-k)!}(j-k)^i \]

\[=\sum_{j=0}^n2^j*j!\sum_{k=0}^{j}\frac{(-1)^k}{k!}*\frac{\sum_{i=0}^{n}(j-k)^i}{(j-k)!} \]

\[g(x)=\frac{(-1)^x}{x!},h(x)=\frac{\sum_{i=0}^nx^i}{x!} \]

\[f(x)=\sum_{j=0}^n2^j*j!\sum_{k=0}^{j}g(k)*h(j-k) \]

\[c(j)=\sum_{k=0}^{j}g(k)*h(j-k) \]

\[f(x)=\sum_{j=0}^n2^j*j!*c(j) \]

#include <cstdio>
#include <algorithm>
using namespace std;

const int pps=998244353,g=3,maxn=262145;

int n,m=1,lg;
int G[maxn],H[maxn],C[maxn];
int bin[maxn],fac[maxn],inv[maxn],rev[maxn];

int read() {
	int x=0,f=1;char ch=getchar();
	for(;ch<'0'||ch>'9';ch=getchar())if(ch=='-')f=-1;
	for(;ch>='0'&&ch<='9';ch=getchar())x=x*10+ch-'0';
	return x*f;
}

int quick(int a,int b) {
	int sum=1;
	while(b) {
		if(b&1)sum=1ll*a*sum%pps;
		a=1ll*a*a%pps;b>>=1;
	}
	return sum;
}

void prepare() {
	H[1]=n+1;
	bin[0]=fac[0]=H[0]=G[0]=inv[0]=1;
	for(int i=1;i<=n;i++)
		bin[i]=1ll*bin[i-1]*2%pps;
	for(int i=1;i<=n;i++)
		fac[i]=1ll*fac[i-1]*i%pps;
	G[n]=inv[n]=quick(fac[n],pps-2);
	for(int i=n-1;i;i--)
		G[i]=inv[i]=1ll*inv[i+1]*(i+1)%pps;
	for(int i=1;i<=n;i+=2)G[i]=pps-G[i];
	for(int i=2;i<=n;i++)
		H[i]=1ll*inv[i]*(quick(i,n+1)-1)%pps*quick(i-1,pps-2)%pps;
}

void NTT(int *a,int sign) {
	for(int i=0;i<m;i++)
		if(rev[i]>i)swap(a[i],a[rev[i]]);
	for(int s=2;s<=m;s<<=1) {
		int gn=quick(g,((pps-1)/s*sign+pps-1)%(pps-1));
		for(int i=0;i<m;i+=s) {
			int w=1;
			for(int j=0;j<(s>>1);j++,w=1ll*w*gn%pps) {
				int x=a[i+j]%pps,y=1ll*w*a[i+(s>>1)+j]%pps;
				a[i+j]=(x+y)%pps,a[i+(s>>1)+j]=(x-y+pps)%pps;
			}
		}
	}
	if(sign==1)return;
	int invm=quick(m,pps-2);
	for(int i=0;i<m;i++)
		a[i]=1ll*a[i]*invm%pps;	
}

int main() {
	n=read();
	prepare();
	for(;m<=n*2;m<<=1,lg++);
	for(int i=0;i<m;i++)
		rev[i]=(rev[i>>1]>>1)|((i&1)<<(lg-1));
	NTT(G,1);NTT(H,1);
	for(int i=0;i<m;i++)
		C[i]=1ll*G[i]*H[i]%pps;
	NTT(C,-1);int ans=1;
	for(int i=1;i<=n;i++)
		ans=(ans+1ll*bin[i]*fac[i]%pps*C[i]%pps)%pps;
	printf("%d\n",ans);
	return 0;
}