Java 解法, 执行用时: 51ms, 内存消耗: 10916K, 提交时间: 2023-08-11 16:50:03

import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
		int []arr = new int[a];
		int res = 0;
		for (int i = 0; i < a; i++)
			arr[i]=sc.nextInt();
		for (int i = 1; i < a; i++) {
			res+=arr[i]*arr[i-1];
			arr[i]+=arr[i-1];
		}
		System.out.println(res);
	}
}

Python3 解法, 执行用时: 40ms, 内存消耗: 4604K, 提交时间: 2023-08-11 16:49:29

n = int(input())
m = [int(v) for v in input().split()]

res = l = 0  # l是前缀和
for j in range(1, n):
    l = m[j-1] + m[j]
    res += m[j-1] * m[j]
    m[j] = l
print(res)