hdu 2993 MAX Average Problem 斜率优化DP

#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
double sum[100010];
int q[100010];
bool judge(int i,int j,int k)
{
   // if((sum[j]-sum[i])/(j-i)>=(sum[k]-sum[i])/(k-i))
   if((sum[j]-sum[i])*(k-i)-(sum[k]-sum[i])*(j-i)>=0)
        return true;            //存在上凸点
    return false;
}
double f(int i,int j)
{
    return (sum[j]-sum[i])/(j-i);
}
inline int ReadInt()
{
    char ch = getchar();
    int data = 0;
    while (ch < '0' || ch > '9')
    {
        ch = getchar();
    }
    do
    {
        data = data*10 + ch-'0';
        ch = getchar();
    }while (ch >= '0' && ch <= '9');
        return data;
}
int main()
{
    int n,k;
    double ans;
    while(scanf("%d%d",&n,&k)!=EOF)
    {
        sum[0]=0;
        for(int i=1;i<=n;i++)
            sum[i]=sum[i-1]+ReadInt();
        int head=0,tail=0;
         ans=0;
         for(int i=k;i<=n;i++)
         {
             while(head<tail && judge(q[tail-1],q[tail],i-k))
                 tail--;
             q[++tail]=i-k;
             while(head<tail && f(q[head+1],i)>=f(q[head],i))
                 head++;
             ans=max(ans,f(q[head],i));
         }
        printf("%.2lf\n",ans);
    }
    return 0;
}