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<iostream>
#include<cstdio>
#include<algorithm>
#include<cstring>
#include<cmath>
#include<queue>
#include<vector>
using namespace std;
#define ll long long 
#define il inline
const int maxn=200010;
const ll mod=1e9+7;
ll a[maxn],s[maxn];
int n; ll ans=0;
int main(){
    scanf("%d",&n);
    for (int i=1;i<=n;++i){
        scanf("%lld",&a[i]);
        s[i]=s[i-1]+a[i];
    }
    for (int i=n;i>=2;--i){
        ans=(ans+s[i-1]*a[i])%mod;
    }
    printf("%lld",ans);
}