傳送門

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題解：

暫時咕咕咕。

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代碼：

#include<bits/stdc++.h> #define ll long long #define re register #define cs constusing std::cerr; using std::cout;cs int mod=998244353; inline int add(int a,int b){a+=b-mod;return a+(a>>31&mod);} inline int dec(int a,int b){a-=b;return a+(a>>31&mod);} inline int mul(int a,int b){ll r=(ll)a*b;return r>=mod?r%mod:r;} inline int power(int a,int b,int res=1){for(;b;b>>=1,a=mul(a,a))(b&1)&&(res=mul(res,a));return res; } inline void Inc(int &a,int b){a+=b-mod;a+=a>>31&mod;} inline void Mul(int &a,int b){a=mul(a,b);}cs int N=105,M=N*1001; int n,k,S; int f[N][M],g[N][M]; int fac[M],ifac[M],inv[M],s[N],c[M]; inline int C(int n,int m){return mul(fac[n],mul(ifac[m],ifac[n-m]));}signed main(){ #ifdef zxyoifreopen("gugugu.in","r",stdin); #endifscanf("%d%d",&n,&k);S=n*k;fac[0]=fac[1]=ifac[1]=ifac[0]=inv[0]=inv[1]=1;for(int re i=2;i<=S;++i){fac[i]=mul(fac[i-1],i);inv[i]=mul(mod-mod/i,inv[mod%i]);ifac[i]=mul(ifac[i-1],inv[i]);}for(int re i=0;i<=n;++i)s[i]=mul(n,inv[n-i]);for(int re i=k-1;i<S;++i)c[i]=C(i,k-1);f[0][0]=1;for(int re i=0;i<n;++i)for(int re j=0;j<S;++j)if(f[i][j]||g[i][j]){int nf=mul(f[i][j],inv[n-i]);int ng=mul(add(g[i][j],mul(f[i][j],s[i])),inv[n-i]);Inc(f[i][j+1],nf);Inc(g[i][j+1],ng);Inc(f[i+1][j+1],mul(nf,c[j-i*k]));Inc(g[i+1][j+1],mul(ng,c[j-i*k]));}cout<<mul(fac[n],g[n][S])<<"\n";return 0; }

總結

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