【斜率优化】HDU 2993 MAX Average Problem

#include<stdio.h>
#include<iostream>
#include<string.h>
#include<queue>
#include<algorithm>
using namespace std;
const int MAXN=100010;

int sum[MAXN];
int q[MAXN];
int top;

long long cross(int a,int b,int c)
{
    long long x1=b-a;
    long long y1=sum[b]-sum[a];
    long long x2=c-b;
    long long y2=sum[c]-sum[b];
    return x1*y2-y1*x2;
}
int bsearch(int l,int r,int i)
{
    while(l<r)
    {
        int mid=(l+r)>>1;
        if(cross(q[mid],q[mid+1],i)<0)r=mid;
        else l=mid+1;
    }
    return l;
}


int input()
{
    char ch=‘ ‘;
    while(ch<‘0‘||ch>‘9‘)ch=getchar();
    int x=0;
    while(ch<=‘9‘&&ch>=‘0‘)x=x*10+ch-‘0‘,ch=getchar();
    return x;
}
int main()
{
   // freopen("in.txt","r",stdin);
   // freopen("out.txt","w",stdout);
    int n,k;
    while(scanf("%d%d",&n,&k)!=EOF)
    {
        top=0;
        sum[0]=0;
        for(int i=1;i<=n;i++)
        {
            sum[i]=input();
            sum[i]+=sum[i-1];
        }
        double ans=0;
        q[top++]=0;
        for(int i=k;i<=n;i++)
        {
            int j=i-k;
            while(top>1&&cross(q[top-2],q[top-1],j)<0)top--;
            q[top++]=j;
            int temp=bsearch(0,top-1,i);
            double f=((double)(sum[i]-sum[q[temp]]))/(i-q[temp]);
            if(f>ans)ans=f;
        }
        printf("%.2lf\n",ans);
    }
    return 0;
}