• P4091 [HEOI2016/TJOI2016]求和(第二类斯特林数+NTT)


    传送门

    首先,因为在(j>i)的时候有(S(i,j)=0),所以原式可以写成$$Ans=sum_{i=0}^nsum_{j=0}^nS(i,j) imes 2^j imes j!$$

    [Ans=sum_{j=0}^n2^j imes j!sum_{i=0}^nS(i,j) ]

    根据第二类斯特林数的通项公式代入,有$$Ans=sum_{j=0}^n2^j imes j!sum_{i=0}^nsum_{k=0}^jfrac{(-1)^k}{k!}frac{(j-k)^i}{(j-k)!}$$

    [Ans=sum_{j=0}^n2^j imes j!sum_{k=0}^jfrac{(-1)^k}{k!}frac{sum_{i=0}^n(j-k)^i}{(j-k)!} ]

    根据等比数列求和公式,知(sum_{i=0}^np^i=frac{p^n-1}{p-1}),于是设(f_i=frac{(-1)^i}{i!},g_i=frac{sum_{k=0}^ni^k}{i!}),则$$Ans=sum_{j=0}^n2^j imes j!(f imes g)(j-k)$$
    (NTT)计算即可

    //minamoto
    #include<bits/stdc++.h>
    #define R register
    #define fp(i,a,b) for(R int i=a,I=b+1;i<I;++i)
    #define fd(i,a,b) for(R int i=a,I=b-1;i>I;--i)
    #define go(u) for(int i=head[u],v=e[i].v;i;i=e[i].nx,v=e[i].v)
    using namespace std;
    char buf[1<<21],*p1=buf,*p2=buf;
    inline char getc(){return p1==p2&&(p2=(p1=buf)+fread(buf,1,1<<21,stdin),p1==p2)?EOF:*p1++;}
    int read(){
        R int res,f=1;R char ch;
        while((ch=getc())>'9'||ch<'0')(ch=='-')&&(f=-1);
        for(res=ch-'0';(ch=getc())>='0'&&ch<='9';res=res*10+ch-'0');
        return res*f;
    }
    char sr[1<<21],z[20];int C=-1,Z=0;
    inline void Ot(){fwrite(sr,1,C+1,stdout),C=-1;}
    void print(R int x){
        if(C>1<<20)Ot();if(x<0)sr[++C]='-',x=-x;
        while(z[++Z]=x%10+48,x/=10);
        while(sr[++C]=z[Z],--Z);sr[++C]='
    ';
    }
    const int N=5e5+5,P=998244353,Gi=332748118;
    inline int add(R int x,R int y){return x+y>=P?x+y-P:x+y;}
    inline int dec(R int x,R int y){return x-y<0?x-y+P:x-y;}
    inline int mul(R int x,R int y){return 1ll*x*y-1ll*x*y/P*P;}
    int ksm(R int x,R int y){
    	R int res=1;
    	for(;y;y>>=1,x=mul(x,x))if(y&1)res=mul(res,x);
    	return res;
    }
    int A[N],B[N],O[N],r[N],fac[N],inv[N];
    int n,lim,l,res;
    void NTT(int *A,int ty){
    	fp(i,0,lim-1)if(i<r[i])swap(A[i],A[r[i]]);
    	for(R int mid=1;mid<lim;mid<<=1){
    		R int I=(mid<<1),Wn=ksm(ty==1?3:Gi,(P-1)/I);O[0]=1;
    		fp(i,1,mid-1)O[i]=mul(O[i-1],Wn);
    		for(R int j=0;j<lim;j+=I)for(R int k=0;k<mid;++k){
    			int x=A[j+k],y=mul(O[k],A[j+k+mid]);
    			A[j+k]=add(x,y),A[j+k+mid]=dec(x,y);
    		}
    	}if(ty==-1)for(R int i=0,inv=ksm(lim,P-2);i<lim;++i)A[i]=mul(A[i],inv);
    }
    int main(){
    //	freopen("testdata.in","r",stdin);
    	scanf("%d",&n);
    	fac[0]=inv[0]=1;fp(i,1,n)fac[i]=mul(fac[i-1],i);
    	inv[n]=ksm(fac[n],P-2);fd(i,n-1,1)inv[i]=mul(inv[i+1],i+1);
    	fp(i,0,n){
    		A[i]=i&1?P-inv[i]:inv[i];
    		if(i!=1)B[i]=mul(dec(ksm(i,n+1),1),mul(inv[i],ksm(dec(i,1),P-2)));
    		else B[i]=n+1;
    		
    	}lim=1;while(lim<=n+n)lim<<=1,++l;
    	fp(i,0,lim-1)r[i]=(r[i>>1]>>1)|((i&1)<<(l-1));
    	NTT(A,1),NTT(B,1);
    	fp(i,0,lim-1)A[i]=mul(A[i],B[i]);
    	NTT(A,-1);
    	for(R int i=0,j=1;i<=n;++i,j=add(j,j))res=add(res,mul(j,mul(fac[i],A[i])));
    	printf("%d
    ",res);return 0;
    }
    
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  • 原文地址:https://www.cnblogs.com/bztMinamoto/p/10210682.html
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