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 Go (1.14.1) (judge verdict: Wrong Answer)

package main

import (
	"bufio"
	"fmt"
	"os"
)

type Node struct {
	Value int64
	Next  *Node
}

func main() {
	r := bufio.NewReader(os.Stdin)
	w := bufio.NewWriter(os.Stdout)
	defer w.Flush()

	var n int64
	fmt.Fscan(r, &n)

	first := &Node{
		Value: 0,
		Next:  nil,
	}

	tmp := first

	var a int64
	for i := int64(0); i < n; i++ {
		fmt.Fscan(r, &a)
		newNode := &Node{
			Value: a,
			Next:  nil,
		}
		tmp.Next = newNode
		tmp = newNode
	}

	var result int64
	var base int64
	base = 1000000000 + 7

	iNode := first.Next
	for iNode.Next != nil {
		jNode := iNode.Next
		for jNode != nil {
			fmt.Printf("%d, %d\n", iNode.Value, jNode.Value)
			r := (iNode.Value * jNode.Value) % base
			result += r
			result %= base
			jNode = jNode.Next
		}
		iNode = iNode.Next
	}

	fmt.Fprintln(w, result)

}