1

 1 // 题目链接  http://codeforces.com/contest/1006/problem/C
 2 #include <iostream>
 3 #include <cstdio>
 4 #include <cstring>
 5 #include <string>
 6 #include <algorithm>
 7 #include <utility>
 8 #include <vector>
 9 #include <map>
10 #include <queue>
11 #include <stack>
12 #include <cstdlib>
13 #include <deque>
14 #include <set>
15 typedef long long ll;
16 #define lowbit(x) (x&(-x))
17 #define ls l,m,rt<<1
18 #define rs m+1,r,rt<<1|1
19 using namespace std;
20 map<ll,ll>mp;
21 map<ll,int>id;
22 int n;
23 const int N=2e5+9;
24 ll a[N];
25 ll sum1[N],sum2[N];
26 int main()
27 {  //cout<<"njjni
";
28     scanf("%d",&n);
29     sum1[0]=0;
30     ll sum=0;
31     for(int i=1;i<=n;i++){
32         scanf("%lld",&a[i]);
33         sum+=a[i];
34         sum1[i]=sum1[i-1]+a[i];
35         mp[sum1[i]]=1;
36         id[sum1[i]]=i;
37     }
38     /*    
39     if(sum%2==0){
40         printf("%lld
",sum/2);
41         return  0;
42     }
43     //上面的是错误的，例如 ：
44     1 1
45     2
46     就应该输出0
47     */
48     sum2[n]=a[n];
49     for(int i=n-1;i>=1;i--){
50         sum2[i]=sum2[i+1]+a[i];
51         
52     }
53     ll maxx=0;
54     int i;
55     for(int j=n;j>=1;j--){      
56         if(mp[sum2[j]]==1){
57             i=id[sum2[j]];
58             if(i<j){
59                 //cout<<i<<" "<<j<<endl;
60             maxx=max(maxx,sum2[j]);
61             }
62         }
63     }
64     printf("%lld
",maxx);
65     return 0;
66 }