P5686 [CSP-S2019 江西] 和积和

#include <bits/stdc++.h>
#define ll long long
using namespace std;
int n;
ll a[550000],b[550000],c[4000],d[4000];
ll ans;
signed main() {
	ios_base::sync_with_stdio(false);
//	freopen("1.in","r",stdin);
    cin >> n;
    for(int i=1;i<=n;i++){
    	cin>>a[i];
	}
	for(int i=1;i<=n;i++){
    	cin>>b[i];
	}
	for(int i=1;i<=n;i++){
		c[i]=c[i-1]+a[i];
		c[i]%=1000000007;
		d[i]=d[i-1]+b[i];
		d[i]%=1000000007;
	}
	for(int l=1;l<=n;l++){
		for(int r=l;r<=n;r++){
			int x=((c[r]-c[l-1])%1000000007)*((d[r]-d[l-1])%1000000007);
			x%=1000000007;
			ans+=x;
			ans%=1000000007;
		}	
	}
	cout<<ans<<endl;
    return 0;
}