数论推公式

 

Print the answer.

Sample Input 1

 

3
2 8 4

Sample Output 1

 

56

We have ${\sum Ni=2\sum i-1j=1(Ai-Aj)^2=(8-2)^2+(4-2)^2+(4-8)^2=56}^{}$.

Sample Input 2

 

5
-5 8 9 -4 -3

Sample Output 2

 

950


这个题是个推公式题，看这个数据范围暴力肯定wa

#include<iostream>
#include<algorithm>
using namespace std;
typedef long long ll;
const int maxn=1e6+100;
int n;
ll a[maxn];
ll ai[maxn];
ll aj[maxn];
ll sum[maxn];
int main(){
    cin>>n;
    for(int i=1;i<=n;i++){
        cin>>a[i];
        sum[i]=sum[i-1]+a[i];
    }
    ll sum1=0;
    ll sum2=0;
    ll sum3=0;
    for(int i=2;i<=n;i++){
        sum1+=(a[i]*a[i]*(i-1));
    }
    for(int i=1;i<=n;i++){
        sum2+=(a[i]*a[i]*(n-i)); 
    }
    for(int i=2;i<=n;i++){
        sum3+=2*(a[i]*sum[i-1]);
    }
    cout<<sum1+sum2-sum3<<endl;
}