第十三届蓝桥杯省赛A组题解传送门

题目大意

给定$n$个整数$a_1$,$a_2$,$\cdots$,$a_n$，求它们两两相乘再相加的和

解题思路

答案等于$a_1\times (a_2+a_3+\cdots +a_n)+a_2\times (a_3+a_4+\cdots +a_n)+\cdots +a_{n-1}\times (a_n)$
容易想到上式在括号里的都用前缀和处理

具体代码

#include <bits/stdc++.h>
typedef long long LL;
const int N = 2e5 + 10;
const int INF = 1e9 + 10;
int a[N];
LL s[N];
int n;
signed main()
{
    std::ios::sync_with_stdio(false);
    std::cin.tie(0);
    std::cout.tie(0);
    std::cin >> n;
    for (int i = 1; i <= n; ++i)
    {
        std::cin >> a[i];
        s[i] = s[i - 1] + a[i];
    }
    LL res = 0;
    for (int i = 1; i <= n - 1; ++i)
        res += a[i] * (s[n] - s[i]);
    std::cout << res << '\n';
    return 0;
}