[藍橋杯 2022 省 A] 求和

暴力破解會超時,用因式分解的平方差公式

a²+ 2ab+b²=(a+)²
a2-2ab+b²=(a-b)²

輸出整數=((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);}
}