正题

luogu-SPOJ 2916

---

题目大意

给你一个序列，有若干询问，每次给出左右端点的区间，问你最大字段和

---

解题思路

用线段树维护区间信息，询问的区间如果有交则分类讨论求解

---

code

#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
#define ll long long
#define N 10100
using namespace std;
int t,n,m,x,y,xx,yy,ans,a[N];
struct node
{int lm,rm,mx,num;
};
node merge(node a,node b)
{node c;c.lm=max(a.lm,a.num+b.lm);c.rm=max(b.rm,a.rm+b.num);c.mx=max(max(a.mx,b.mx),a.rm+b.lm);c.num=a.num+b.num;return c;
}
struct Tree
{#define ls x*2#define rs x*2+1node v[N<<2];void push_up(int x){v[x]=merge(v[ls],v[rs]);return;}void build(int x,int l,int r){if(l==r){v[x]={a[l],a[l],a[l],a[l]};return;}int mid=l+r>>1;build(ls,l,mid);build(rs,mid+1,r);push_up(x);return;}node ask(int x,int L,int R,int l,int r){if(L==l&&R==r)return v[x];int mid=L+R>>1;if(r<=mid)return ask(ls,L,mid,l,r);else if(l>mid)return ask(rs,mid+1,R,l,r);else return merge(ask(ls,L,mid,l,mid),ask(rs,mid+1,R,mid+1,r));}
}T;
int main()
{scanf("%d",&t);while(t--){scanf("%d",&n);for(int i=1;i<=n;++i)scanf("%d",&a[i]);T.build(1,1,n);scanf("%d",&m);while(m--){scanf("%d%d%d%d",&x,&y,&xx,&yy);if(y<xx){ans=T.ask(1,1,n,x,y).rm+T.ask(1,1,n,xx,yy).lm;if(y+1<=xx-1)ans+=T.ask(1,1,n,y+1,xx-1).num;printf("%d\n",ans);}else{ans=T.ask(1,1,n,xx,y).mx;if(x<xx)ans=max(ans,T.ask(1,1,n,x,xx-1).rm+T.ask(1,1,n,xx,y).lm);if(y<yy)ans=max(ans,T.ask(1,1,n,xx,y).rm+T.ask(1,1,n,y+1,yy).lm);if(x<xx&&y<yy)ans=max(ans,T.ask(1,1,n,x,xx-1).rm+T.ask(1,1,n,xx,y).num+T.ask(1,1,n,y+1,yy).lm);printf("%d\n",ans);}}}return 0;
}