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"
	"math"
	"os"
	"strconv"
)

func nextInt() int {
	sc.Scan()
	i, e := strconv.Atoi(sc.Text())
	if e != nil {
		panic(e)
	}
	return i
}

var sc = bufio.NewScanner(os.Stdin)

func main() {
	// Code for C - Sum of product of pairs
	var N int
	fmt.Scanf("%d", &N)

	sc.Split(bufio.ScanWords)
	A := make([]int, N)
	var sum float64
	for i := 0; i < N; i++ {
		A[i] = nextInt()
		sum += float64(A[i])
		sum = float64(int(sum) % (int(math.Pow10(9)) + 7))
	}

	//fmt.Println(sum)
	var ans int
	for i := 0; i < N-1; i++ {
		sum -= float64(A[i])
		//fmt.Println(sum)
		ans += (A[i] * int(sum)) % (int(math.Pow10(9)) + 7)
		ans = ans % (int(math.Pow10(9)) + 7)
		//fmt.Println(sum, ans)
		//for j := i + 1; j < N; j++ {
		//	ans += A[i] * A[j] % (int(math.Pow10(9)) + 7)
		//	ans = ans % (int(math.Pow10(9)) + 7)
		//}
	}

	fmt.Println(ans)
}