Problem:
Score : 300 points

Problem Statement

Given are N integers A_1,\ldots,A_N.

Find the sum of A_i \times A_j over all pairs (i,j) such that 1\leq i < j \leq N, modulo (10^9+7).

Constraints

2 \leq N \leq 2\times 10^5

0 \leq A_i \leq 10^9

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N
A_1 \ldots A_N

Output

Print \sum_{i=1}^{N-1}\sum_{j=i+1}^{N} A_i A_j, modulo (10^9+7).

Sample Input 1

3
1 2 3

Sample Output 1

11

We have 1 \times 2 + 1 \times 3 + 2 \times 3 = 11.

Sample Input 2

4
141421356 17320508 22360679 244949

Sample Output 2

437235829

Solution in C++ (GCC 9.2.1) (judge verdict: Wrong Answer)

    #include <bits/stdc++.h>
    using namespace std;
    #define ll long long
    #define pb push_back
    #define NUM 1000000007
    #define MAX 100001
    #define INF LLONG_MAX
    int main(){
        ios_base::sync_with_stdio(false);
        cin.tie(NULL);
        cout.tie(NULL);
        ll n,tmp;
        cin>>n;
        vector<ll> v;
        for(int i=0;i<n;i++){
            cin>>tmp;
            v.pb(tmp);
        }
        vector<ll> prf(n+1);
        prf[0]=v[0];
        for(int i=1;i<n;i++){
            prf[i]=prf[i-1]+v[i];
        }
        ll ans = 0;
        for(int i=0;i<n-1;i++){
            ans=(ans+(v[i]*(prf[n-1]-prf[i])))%NUM;
        }
        cout<<ans<<endl;
        return 0;
    }