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: Accepted)

package main

import (
	"bufio"
	"fmt"
	"math/big"
	"os"
	"strconv"
)

type splitter int

const (
	bySpace splitter = iota
	byLine
)

func newScanner(sp splitter) *sc {
	scanner := bufio.NewScanner(os.Stdin)
	if sp == bySpace {
		scanner.Split(bufio.ScanWords)
	}
	return &sc{
		s: scanner,
	}
}

type sc struct {
	s *bufio.Scanner
}

func (s *sc) nextInt() int {
	if s.s.Scan() {
		val := s.s.Text()
		v, err := strconv.Atoi(val)
		if err != nil {
			panic(err)
		}
		return v
	}
	panic("no more stdin")
}

func main() {
	scanner := newScanner(bySpace)
	n := scanner.nextInt()
	arr := make([]int64, n)
	var tot int64 = 0
	for i := 0; i < n; i++ {
		p := scanner.nextInt()
		tot += int64(p)
		arr[i] = int64(p)
	}
	minus := big.NewInt(0)
	ans := big.NewInt(0)
	m := big.NewInt(1000000007)
	bigTot := big.NewInt(tot)
	for _, v := range arr {
		cur := big.NewInt(v)
		minus = big.NewInt(0).Add(minus, cur)
		buf := big.NewInt(0).Mul(cur, big.NewInt(0).Sub(bigTot, minus))
		ans = big.NewInt(0).Mod(big.NewInt(0).Add(ans, buf), m)
	}
	fmt.Println(ans.Int64())
}