CF1279D Santa's Bot

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普及题，开个桶记录一下每个数出现的次数即可。

#include<cstdio>
#include<cctype>
#include<vector>
namespace IO
{
    char ibuf[(1<<21)+1],*iS,*iT;
    char Get(){return (iS==iT? (iT=(iS=ibuf)+fread(ibuf,1,(1<<21)+1,stdin),(iS==iT? EOF:*iS++)):*iS++);}
    int read(){int x=0,c=Get();while(!isdigit(c))c=Get();while(isdigit(c))x=x*10+c-48,c=Get();return x;}
}
using IO::read;
const int N=1000007,P=998244353;
std::vector<int>a[N];
int k[N],cnt[N];
void mod(int&x){x-=P,x+=(x>>31&P);}
int pow(int a,int k){int r=1;for(;k;k>>=1,a=1ll*a*a%P)if(k&1)r=1ll*a*r%P;return r;}
int inv(int a){return pow(a,P-2);}
int main()
{
    int n=read(),in=inv(n),ans=0;
    for(int i=1,j,x;i<=n;++i) for(j=1,k[i]=read();j<=k[i];++j) ++cnt[x=read()],a[i].push_back(x);
    for(int i=1;i<=n;++i)
    {
    int sum=0;
    for(int x:a[i]) mod(sum+=1ll*cnt[x]*in%P);
    mod(ans+=1ll*sum*inv(k[i])%P);
    }
    printf("%d",1ll*ans*in%P);
}

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来源： https://www.cnblogs.com/cjoierShiina-Mashiro/p/12243587.html