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[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);}
}