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 (
	"fmt"
)

func main() {
	var n int
	nums := []int64{}

	_, err := fmt.Scanf("%d", &n)
	if err != nil {
		return
	}
	if n <= 1 {
		fmt.Printf("0\n")
		return
	}

	for i:=0;i<n;i++{
		var tn int64
		_, err := fmt.Scanf("%d", &tn)
		if err != nil {
			return
		}

		nums = append(nums, tn)
	}

	//开始真正的逻辑
	length := len(nums)

	sums := make([]int64, length)  //用来加速运算
	sums[length-1] = nums[length-1]
	for i:=len(sums)-2;i>=0;i--{
		sums[i] = sums[i+1] + nums[i]
	}

	var sum int64
	for i:=0;i+1<length;i++ {
		sum += nums[i] * (sums[i] - nums[i])
		sum = sum % 1000000007
	}

	fmt.Printf("%d\n", sum)
}