每日一题蓝桥杯P8772 [蓝桥杯 2022 省 A] 求和 题解c++

#include
using namespace std;
int main() {
	int t;
	int n;
	cin >> n;
	long long sum = 0;
	long long hpf = 0;
	long long pfh = 0;
	for (int i = 0; i < n; i++) {
		cin >> t;
		pfh += t * t;
		hpf += t;
	}
	hpf = hpf * hpf;
	sum = (hpf - pfh) / 2;
	cout << sum << endl;
	return 0;
}