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: Time Limit Exceeded)

#include <bits/stdc++.h>
using namespace std;
int main()
{
   int n;
   const int mod = 1000000007;
   cin >> n;
   long long  a[n];
   long long ans=0;
   for(int i = 0; i < n; ++i) {
     cin >> a[i];
   }
   for(int i = 0; i < n-1; ++i) {
     for(int j = i+1; j < n; ++j) {
       ans = (ans + (a[i] * a[j] % mod)) % mod;
     }
   }
   cout << ans << endl;
}