http://www.spoj.com/problems/GSS1/

My Code 
 1 #include <cstdio>
 2 #include <algorithm>
 3 using namespace std;
 4 #define lson l,m,rt<<1
 5 #define rson m+1,r,rt<<1|1
 6 #define maxn 50010
 7 struct node
 8 {
 9     int lmax,rmax,max,sum;
10 }setree[maxn<<2];
11 int ans;
12 void pushup(int rt)
13 {
14     setree[rt].lmax=max(setree[rt<<1].lmax,setree[rt<<1].sum+setree[rt<<1|1].lmax);
15     
16     setree[rt].rmax=max(setree[rt<<1|1].rmax,setree[rt<<1|1].sum+setree[rt<<1].rmax);
17     
18     setree[rt].sum=setree[rt<<1].sum+setree[rt<<1|1].sum;
19     
20     setree[rt].max=max(setree[rt<<1].max,setree[rt<<1|1].max);
21     setree[rt].max=max(setree[rt].max,setree[rt<<1].rmax+setree[rt<<1|1].lmax);
22 }
23 void build(int l,int r,int rt)
24 {
25     if(l==r){
26         scanf("%d",&setree[rt].sum);
27         setree[rt].lmax=setree[rt].sum;
28         setree[rt].rmax=setree[rt].sum;
29         setree[rt].max=setree[rt].sum;
30         return;
31     }
32     int m=(l+r)>>1;
33     build(lson);
34     build(rson);
35     pushup(rt);
36 }
37 int query(int l,int r,int rt,int flag,int L,int R)
38 {
39     if(L==l&&r==R){
40         ans=max(ans,setree[rt].max);
41         return flag==-1?setree[rt].lmax:setree[rt].rmax;
42     }
43     int m=(l+r)>>1;
44     if(R<=m)
45     return query(lson,-1,L,R);
46     else if(L>m)
47     return query(rson,1,L,R);
48     else{
49         int ln=query(lson,1,L,m);
50         int rn=query(rson,-1,m+1,R);
51         ans=max(ans,ln+rn);
52         if(flag == -1)
53             return max(setree[rt<<1].lmax, setree[rt<<1].sum + rn);
54         else
55             return max(setree[rt<<1|1].rmax, setree[rt<<1|1].sum + ln);
56 
57     }
58     return ans;
59 }
60 int main()
61 {
62     int n;
63     while(~scanf("%d",&n)){
64         build(1,n,1);
65         int m;
66         scanf("%d",&m);
67         while(m--){
68             int a,b;
69             ans=-(1<<30);
70             scanf("%d%d",&a,&b);
71             int tmp=query(1,n,1,0,a,b);
72             printf("%d\n",ans);
73         }
74     }
75     return 0;
76 }