Iahub accidentally discovered a secret lab. He found there n devices ordered in a line, numbered from 1 to n from left to right. Each device i (1 ≤ i ≤ n) can create either ai units of matter or ai units of antimatter.

Iahub wants to choose some contiguous subarray of devices in the lab, specify the production mode for each of them (produce matter or antimatter) and finally take a photo of it. However he will be successful only if the amounts of matter and antimatter produced in the selected subarray will be the same (otherwise there would be overflowing matter or antimatter in the photo).

You are requested to compute the number of different ways Iahub can successful take a photo. A photo is different than another if it represents another subarray, or if at least one device of the subarray is set to produce matter in one of the photos and antimatter in the other one.

The first line contains an integer n (1 ≤ n ≤ 1000). The second line contains n integers a1, a2, ..., an (1 ≤ ai ≤ 1000).

The sum a1 + a2 + ... + an will be less than or equal to 10000.

Output a single integer, the number of ways Iahub can take a photo, modulo 1000000007 (109 + 7).

The possible photos are [1+, 2-], [1-, 2+], [2+, 3-], [2-, 3+], [3+, 4-], [3-, 4+], [1+, 2+, 3-, 4-], [1+, 2-, 3+, 4-], [1+, 2-, 3-, 4+], [1-, 2+, 3+, 4-], [1-, 2+, 3-, 4+] and [1-, 2-, 3+, 4+], where "i+" means that the i-th element produces matter, and "i-" means that the i-th element produces antimatter.

dp 背包问题

 #include <iostream>

 #include <cstdio>

 #include <cstring>

 #include <algorithm>

 using namespace std;

 const int MAX_N = ;

 const int MOD = 1e9 + ;

 int N,_max = ;

 int a[MAX_N],dp[MAX_N][];

 void solve() {

         for(int i = ; i <= N; ++i) {

                 for(int j =  * _max; j >= ; --j) {

                         //dp[i][j] = dp[i - 1][j];

                         int v = ;

                         if(j - a[i] ==  + _max) v = ;

                         dp[i][j] = (dp[i][j] + v + dp[i - ][j - a[i]]) % MOD;

                         dp[i][j] = (dp[i][j] + v + dp[i - ][j + a[i]]) % MOD;

                 }

         }

         int ans = ;

        // printf("dds = %d %d\n",dp[3][5],dp[3][3]);

         for(int i = ; i <= N; ++i) ans = (ans + dp[i][ + _max]) % MOD;

         printf("%d\n",ans);

 }

 int main()

 {

     scanf("%d",&N);

     for(int i = ; i <= N; ++i) {

             scanf("%d",&a[i]);

             //printf("%d \n",a[i]);

             _max += a[i];

     }

     solve();

     return ;

 }