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 Kotlin (1.3.71) (judge verdict: Wrong Answer)

import java.io.BufferedReader
import java.io.IOException
import java.io.InputStreamReader
import java.util.*
import kotlin.math.*

fun main() {
    var fs = Atcoder.FastReader()
    var n = fs.nextInt()
    var a = fs.readLongArray(n)
    var sum:Long = 0
    for (i in 0 until a.size) {
        sum += a[i]
    }
    var ans:Long = 0
    for (i in 0..a.size - 2) {
        sum -= a[i]
        ans += a[i]*sum
        ans %= 1000000007
    }
    println(ans)


}

class Atcoder {
    internal class FastReader {
        private var br: BufferedReader = BufferedReader(InputStreamReader(System.`in`))
        private var st: StringTokenizer? = null

        operator fun next(): String {
            while (st == null || !st!!.hasMoreElements()) {
                try {
                    st = StringTokenizer(br.readLine())
                } catch (e: IOException) {
                    e.printStackTrace()
                }

            }
            return st!!.nextToken()
        }

        fun nextInt(): Int {
            return Integer.parseInt(next())
        }

        fun nextLong(): Long {
            return java.lang.Long.parseLong(next())
        }

        fun nextDouble(): Double {
            return java.lang.Double.parseDouble(next())
        }

        fun nextLine(): String {
            var str = ""
            try {
                str = br.readLine()
            } catch (e: IOException) {
                e.printStackTrace()
            }
            return str
        }

        fun readIntArray(n: Int): IntArray {
            val a = IntArray(n)
            for (i in 0 until n) a[i] = nextInt()
            return a
        }

        fun readLongArray(n: Int): LongArray {
            val a = LongArray(n)
            for (i in 0 until n) a[i] = nextLong()
            return a
        }

        fun readDoubleArray(n: Int): DoubleArray {
            val a = DoubleArray(n)
            for (i in 0 until n) a[i] = nextDouble()
            return a
        }
    }
}