题目链接：http://acm.hdu.edu.cn/showproblem.php?pid=5073

Galaxy

Good news for us: to release the financial pressure, the government started selling galaxies and we can buy them from now on! The first one who bought a galaxy was Tianming Yun and he gave it to Xin Cheng as a present.


To be fashionable, DRD also bought himself a galaxy. He named it Rho Galaxy. There are n stars in Rho Galaxy, and they have the same weight, namely one unit weight, and a negligible volume. They initially lie in a line rotating around their center of mass.

Everything runs well except one thing. DRD thinks that the galaxy rotates too slow. As we know, to increase the angular speed with the same angular momentum, we have to decrease the moment of inertia.

The moment of inertia I of a set of n stars can be calculated with the formula


where w_i is the weight of star i, d_i is the distance form star i to the mass of center.

As DRD’s friend, ATM, who bought M78 Galaxy, wants to help him. ATM creates some black holes and white holes so that he can transport stars in a negligible time. After transportation, the n stars will also rotate around their new center of mass. Due to financial pressure, ATM can only transport at most k stars. Since volumes of the stars are negligible, two or more stars can be transported to the same position.

Now, you are supposed to calculate the minimum moment of inertia after transportation.

2
3 2
-1 0 1
4 2
-2 -1 1 2

0
0.5

#include<iostream>
#include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
#define MAXN 50100
using namespace std;
int T,N,K;
int pos[MAXN],sum[MAXN],x[MAXN],First,Last;
double DL,DR,mass;
int PL,PR,need;
double ans,NowI;
double GetNext(){
    double ret=NowI;
    double pmass=mass;
    mass=(sum[Last+1]-sum[First]+0.0)/need;
    NowI+=-(pmass-pos[First])*(pmass-pos[First])+(mass-pos[Last+1])*(mass-pos[Last+1]);
    NowI+=(mass-pmass)*(mass-pmass)*(need-1);
    DL-=(pmass-pos[First]);
    Last++;First++;
    int t=PL-First+1;
    NowI+=2*DL*(mass-pmass);
    DL+=(mass-pmass)*t;
    PL++;
    while(PL<=N && pos[PL]<=mass){
        DL+=fabs(mass-pos[PL]);
        DR-=fabs(pmass-pos[PL]);
        NowI+=(mass-pos[PL])*(mass-pos[PL]);
        NowI-=(pmass-pos[PL])*(pmass-pos[PL]);
        NowI-=(mass-pmass)*(mass-pmass);
        PL++;
    }
    NowI+=2*(pmass-mass)*DR;
    DR+=(pos[Last]-mass);
    PL--;
    DR-=(mass-pmass)*(Last-(PL+1));
    return NowI;
}
int main(){
//    freopen("in.txt","r",stdin);
    scanf("%d",&T);
    while(T--){
        scanf("%d%d",&N,&K);
        for(int i=0;i<N;i++)
            scanf("%d",&pos[i]);
        sort(pos,pos+N);
        sum[0]=pos[0];
        for(int i=1;i<N;i++)
            sum[i]=sum[i-1]+pos[i];
        need=N-K;ans=0;PL=0;
        if(N==1 || need <2 ){
            puts("0.000000000000");
            continue;
        }
        double kkk=sum[need-1]*1.0/need;First=0;Last=need-1;
        DL=0;PL=0;DR=0;NowI=0;
        for(int i=0;i<need;i++){
            NowI+=(kkk-pos[i])*(kkk-pos[i]);
            if(pos[i]<=kkk){
                PL=i;
                DL+=(kkk-pos[i]);
            }
            else
                DR+=(pos[i]-kkk);
        }
        mass=kkk;
        ans=NowI;
        while(Last<N-1){
            GetNext();
            ans=min(ans,NowI);
        }
        printf("%.12lf\n",ans);
    }
}

hdu 5073 Galaxy（2014acm亚洲赛区鞍山 C）

原文：http://blog.csdn.net/asdfghjkl1993/article/details/40398831