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: Time Limit Exceeded)

import kotlin.math.pow

fun main(args: Array<String>) {
	readLine()
	val aList = readLine()?.split(" ")?.map {it.toLong()} ?: return
	
	var sum: Long = 0
	var tempSum: Long = 0
	for (i in aList.indices) {
		for (j in (i+1) until aList.size) {
			tempSum += aList[j]
		}
		tempSum %= (10.0.pow(9)+7).toLong()
		sum = (sum + (aList[i] * tempSum)) % ((10.0.pow(9)+7).toLong())
		tempSum = 0
	}
	
	println(sum)
}