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 Scala (2.13.1) (judge verdict: Wrong Answer)

import scala.io.StdIn.{readInt, readLine}
import scala.collection.mutable.ListBuffer

object Main extends App {

  val n = readInt()
  val in = readLine().split(' ').map(_.toInt)
  val a = ListBuffer.fill(n)(0)
  val b = ListBuffer.fill(n + 1)(0L)
  val mod = 1000000007

  for (i <- 0 until n) {
    a(i) += in(i)
    b(i + 1) += b(i) + a(i)
  }

  var ans = 0L

  for (i <- 0 until n) {
    val sum = b(n) - b(i + 1) % mod

    ans += a(i) * sum
    ans %= mod
  }
  println(ans)
}