[蓝桥杯 2022 省 A] 求和

暴力破解会超时,用因式分解的平方差公式

a²+ 2ab+b²=(a+)²
a²-2ab+b²=(a-b)²

输出整数=((a1+a2+a3+…+an)²-a1²-a2²-a3²-…-an²)/2

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        long n = scanner.nextLong();
        long mul = 0, sum = 0;

        for (long i = 1; i <= n; i++) {
            long x = scanner.nextLong();
            sum += (x * x);
            mul += x;
        }

        System.out.println((mul * mul - sum) / 2);
    }
}