bzoj 2428: [HAOI2006]均分数据

 #include<cstdio>
 #include<iostream>
 #include<cstdlib>
 #include<ctime>
 #include<cstring>
 #include<cmath>
 #include<algorithm>
 #define M 100
 using namespace std;
 int n,m,a[M],pos[M];
 double rev,sum[M],ans,mn=1e30,pre;
 void solve()
 {
     memset(sum,,sizeof(sum));
     ans=;
     for(int i=;i<=n;i++)
       pos[i]=rand()%m+;
     for(int i=;i<=n;i++)
       sum[pos[i]]+=a[i];
     for(int i=;i<=m;i++)
       ans+=(sum[i]-rev)*(sum[i]-rev);
     double T=;
     for(;T>0.1;)
       {
         int t1=rand()%n+,x=pos[t1],y=rand()%m+;
         if(x==y)
           continue;
         T*=0.9;
         pre=ans;
         ans-=(sum[x]-rev)*(sum[x]-rev);
         ans-=(sum[y]-rev)*(sum[y]-rev);
         sum[x]-=a[t1];
         sum[y]+=a[t1];
         ans+=(sum[x]-rev)*(sum[x]-rev);
         ans+=(sum[y]-rev)*(sum[y]-rev);
         if(ans>pre)
           {
             ans=pre;
             sum[x]+=a[t1];
             sum[y]-=a[t1];
           }
         else
           pos[t1]=y;
       }
     if(ans<mn)
       mn=ans;
     return;
 }
 int main()
 {
     scanf("%d%d",&n,&m);
     for(int i=;i<=n;i++)
     {
     scanf("%d",&a[i]);
     rev+=a[i];
     swap(a[i],a[rand()%i+]);
     }
     rev/=m;
     for(int i=;i<=;i++)solve();
     printf("%.2lf",sqrt(mn/m));
     return ;
 }

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