第十三届蓝桥杯省赛C++A/C组——求和

#include 

using namespace std;

typedef long long LL;

int main()
{
    int n;
    scanf("%d", &n);
    LL sum1 = 0, sum2 = 0;
    while (n -- )
    {
        int x;
        scanf("%d", &x);
        sum1 += x, sum2 += x * x;
    }
    printf("%lld\n", (sum1 * sum1 - sum2) / 2);
    
    return 0;
    
}

#include 

using namespace std;

typedef long long LL;

const int N = 200010;

int a[N];
LL s[N];
int n;

int main()
{
    scanf("%d", &n);
    for (int i = 1; i <= n; i ++ ) cin >> a[i], s[i] = s[i - 1] + a[i];
    
    LL sum = 0;
    for (int i = 1; i <= n; i ++ ) 
        sum += (LL)a[i] * (s[n] - s[i]);
        
    printf("%lld\n", sum);
    return 0;
}