POJ 2018 Best Cow Fences（斜率优化+DP）

题目链接

和HDU 2993一样，两个题都很纠结啊。这个题调了好几天。。。终于在多次WA终于AC了。各种错误，判断条件写错，dp数组的初始值。HDU那个题还要手写读入，890ms水过了。这个题放到图上很直观了，网上有各种解释。。。

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#define N  500001
__int64 dp[N],que[N],p[N],sum[N];
int i,m;
double slope(int x,int y)
{
    return ((i+m-y)*sum[x-1]-(i-x+m)*sum[y-1])*1.0/(x-y);
}
int main()
{
    int n,str,end;
    __int64 max;
    scanf("%d%d",&n,&m);

    for(i = 1; i <= n; i ++)
    {
        scanf("%I64d",&p[i]);
        sum[i] = p[i]+sum[i-1];
    }
    str = end = 0;
    for(i = 1; i <= n-m+1; i ++)
    {
        dp[i+m-1] = 1000*(sum[i+m-1] - sum[i-1])/m;
        while(str < end&& sum[i+m-1]>= slope(que[str],que[str+1]))
            str ++;
        if(dp[i+m-1] < 1000*(sum[i+m-1] - sum[que[str]-1])/(i+m-que[str]))
            dp[i+m-1] = 1000*(sum[i+m-1] - sum[que[str]-1])/(i+m-que[str]);
        while(str < end&&slope(que[end-1],que[end]) >= slope(que[end],i))
            end --;
        end ++;
        que[end] = i;
    }
    max = 0;
    for(i = m; i <= n; i ++)
        if(max < dp[i])
            max = dp[i];
    printf("%I64d\n",max);
    return 0;
}