Potted Flower(线段树+dp)

http://poj.org/problem?id=2750

题意：在一个圈中取若干个相邻的数，求他们的最大序列和。不能够同时取所有的数。

看了一篇解题报告写的很详细。。http://blog.csdn.net/non_cease/article/details/7437690

 #include <stdio.h>
 #include <stdlib.h>
 #include <algorithm>
 #include <string.h>
 const int N=;
 using namespace std;
 struct node
 {
     int l,r,sum,minsum,maxsum;
     int lmax,rmax,lmin,rmin;
 } Tree[*N];
 int a[N];
 void pushup(int rt)
 {
     int l = *rt;
     int r = *rt+;
     Tree[rt].sum=Tree[l].sum+Tree[r].sum;
     Tree[rt].minsum=min(min(Tree[l].minsum,Tree[r].minsum),Tree[l].rmin+Tree[r].lmin);
     Tree[rt].maxsum=max(max(Tree[l].maxsum,Tree[r].maxsum),Tree[l].rmax+Tree[r].lmax);
     Tree[rt].lmax=max(Tree[l].lmax,Tree[l].sum+Tree[r].lmax);
     Tree[rt].rmax=max(Tree[r].rmax,Tree[r].sum+Tree[l].rmax);
     Tree[rt].lmin=min(Tree[l].lmin,Tree[l].sum+Tree[r].lmin);
     Tree[rt].rmin=min(Tree[r].rmin,Tree[r].sum+Tree[l].rmin);
 }
 void build(int l,int r,int rt)
 {
     Tree[rt].l = l;
     Tree[rt].r = r;
     if (l==r)
     {
         Tree[rt].sum=Tree[rt].minsum=Tree[rt].maxsum=a[r];
         Tree[rt].lmax=Tree[rt].rmax=Tree[rt].lmin=Tree[rt].rmin=a[r];
         return ;
     }
     int mid = (l+r)>>;
     build(l,mid,*rt);
     build(mid+,r,*rt+);
     pushup(rt);
 }
 void update(int pos,int val,int rt)
 {
     if (Tree[rt].l==Tree[rt].r)
     {
         Tree[rt].sum=Tree[rt].minsum=Tree[rt].maxsum=val;
         Tree[rt].lmax=Tree[rt].rmax=Tree[rt].lmin=Tree[rt].rmin=val;
         return ;
     }
     int mid=(Tree[rt].l+Tree[rt].r)>>;
     if (pos<=mid)
         update(pos,val,*rt);
     else
         update(pos,val,*rt+);
     pushup(rt);
 }
 int main()
 {
     int n,m,pos,val;
     scanf("%d",&n);
     for (int i = ; i <=n; i++)
     {
         scanf("%d",&a[i]);
     }
     build(,n,);
     scanf("%d",&m);
     while(m--)
     {
         scanf("%d%d",&pos,&val);
         update(pos,val,);
         if(Tree[].sum==Tree[].maxsum)
         {
             printf("%d\n",Tree[].sum-Tree[].minsum);
         }
         else
         {
             printf("%d\n",max(Tree[].maxsum,Tree[].sum-Tree[].minsum));
         }
     }
     return ;
 }