2017-9-3 校内模拟T2取数win

#include <iostream>
#include <cstdio>
#include <algorithm>
using namespace std;
int n;
long long lefts,rights,lm,mr;
int num[200100],presum[200100];
double ansx,anss,ans,hh=-1;
double df(long long w,long long l){
    long long k=presum[w]-presum[w-l-1]+presum[n]-presum[n-l];
    return (double)k/(double)(2*l+1)-(double)num[w];
}
int main(){
    //freopen("win.in","r",stdin);
    //freopen("win.out","w",stdout); 
    scanf("%d",&n);
    if(n<3){
        printf("0.00\n");
        return 0;
    }
    for (int i=1;i<=n;i++)
        scanf("%d",&num[i]);
    presum[0]=1;
    sort(num+1,num+n+1);
    for (int i=1;i<=n;i++)
        presum[i]=presum[i-1]+num[i];
    for (int i=1;i<=n;i++){
        lefts=0;
        rights=min(n-1,i-1);
        while(lefts<rights){
            lm=(rights-lefts)/3+lefts;
            mr=rights-(rights-lefts)/3;
            ansx=df(i,lm);anss=df(i,mr);
            if(ansx>anss) rights=mr-1;
            else lefts=lm+1;
        } 
        ans=df(i,lefts);
        if(ans>hh)
            hh=ans;
    }
    printf("%.2lf\n",hh);
    return 0;
}