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。。。

 1 #include <stdio.h>

 2 #include <string.h>

 3 #include <stdlib.h>

 4 #define N  500001

 5 __int64 dp[N],que[N],p[N],sum[N];

 6 int i,m;

 7 double slope(int x,int y)

 8 {

 9     return ((i+m-y)*sum[x-1]-(i-x+m)*sum[y-1])*1.0/(x-y);

10 }

11 int main()

12 {

13     int n,str,end;

14     __int64 max;

15     scanf("%d%d",&n,&m);

16 

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

18     {

19         scanf("%I64d",&p[i]);

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

21     }

22     str = end = 0;

23     for(i = 1; i <= n-m+1; i ++)

24     {

25         dp[i+m-1] = 1000*(sum[i+m-1] - sum[i-1])/m;

26         while(str < end&& sum[i+m-1]>= slope(que[str],que[str+1]))

27             str ++;

28         if(dp[i+m-1] < 1000*(sum[i+m-1] - sum[que[str]-1])/(i+m-que[str]))

29             dp[i+m-1] = 1000*(sum[i+m-1] - sum[que[str]-1])/(i+m-que[str]);

30         while(str < end&&slope(que[end-1],que[end]) >= slope(que[end],i))

31             end --;

32         end ++;

33         que[end] = i;

34     }

35     max = 0;

36     for(i = m; i <= n; i ++)

37         if(max < dp[i])

38             max = dp[i];

39     printf("%I64d\n",max);

40     return 0;

41 }