hdu 2993 斜率优化+二分查找

#include<stdio.h>

#include<iostream>

#include<string.h>

#include<queue>

#include<algorithm>

#define ll long long

using namespace std;

const int MAXN=100010;

ll 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()

{

int n,k;

while(scanf("%d%d",&n,&k)!=EOF)

{

top=0;

sum[0]=0;

for(int i=1;i<=n;i++)

{

sum[i]=(ll)input();

sum[i]+=sum[i-1];

}

double ans=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;

}