[hdu][2993][MAX Average Problem]

题目：http://acm.hdu.edu.cn/showproblem.php?pid=2993

一个细节没注意，导致WA了无数遍，不过也让我对斜率优化有了更深的认识，要维护“队列里”的数据的凸性。

#include <iostream>
#include <stdio.h>
#include <string.h>
#include <algorithm>

using namespace std;

const int N = 100000+10;

int n, m, l, r, k;
int q[N];
double a[N];
char ch = 0;

void input(){
    a[0] = 0;
    ch = 0;
    int s=1;
    while (s<=n){
        int sum = 0;
        while (ch>'9'||ch<'0') ch = getchar();
        while (ch<='9'&&ch>='0') sum=sum*10+ch-'0',ch=getchar();
        a[s]=a[s-1]+sum, s++;
    }
}

bool superior1(int k1, int k2, int r)
{
    return (a[k2]-a[k1])*(r-k2)<(a[r]-a[k2])*(k2-k1);
}

bool superior2(int k1, int k2, int r)
{
    return (a[r]-a[k1])*(r-k2)<=(a[r]-a[k2])*(r-k1);
}

int main()
{
    //freopen("/home/nigel/project/a", "r", stdin);
    while (~scanf("%d%d", &n, &k))
    {
        input();
        double ans = 0;
        q[l=r=0] = 0;
        for (int i=k; i<=n; i++)
        {
            int now = i-k;
            while (l<r&&!superior1(q[r-1],q[r],now)) r--;  //这里把now写成了i，各种WA
            q[++r]=now;
            while (l<r&&superior2(q[l],q[l+1],i)) l++;
            ans = max(ans, (a[i]-a[q[l]])/(i-q[l]));
        }
        printf("%.2lf\n", ans);
    }
    return 0;
}