题面里的
by wdd2929 @ 2023-08-27 16:58:34
@[wdd2929](/user/672940) 举例：$n=6$
```
ANS=S(1,1)+S(1,2)+S(1,3)+S(1,4)+S(1,5)+S(1,6)
+S(2,2)+S(2,3)+S(2,4)+S(2,5)+S(2,6)
+S(3,3)+S(3,4)+S(3,5)+S(3,6)
+S(4,4)+S(4,5)+S(4,6)
+S(5,5)+S(5,6)
+S(6,6)
```
by wpy233 @ 2023-08-27 17:01:21
@[wpy233](/user/214172) 哦，谢谢大佬指点
by wdd2929 @ 2023-08-27 17:02:16
@[wpy233](/user/214172) 大佬
感觉代码没啥问题结果样例也过不去
```cpp
// Problem:
// P5686 [CSP-S2019 江西] 和积和
//
// Contest: Luogu
// URL: https://www.luogu.com.cn/problem/P5686
// Memory Limit: 125 MB
// Time Limit: 1000 ms
//
// Powered by CP Editor (https://cpeditor.org)
#include<iostream>
#include<algorithm>
//#include<cstdio>
#define ll long long
#define endl '\n'
#define rep(i,a,b) for(int i=(a);i<=(b);i++)
#define per(i,a,b) for(int i=(a);i>=(b);i--)
#define N 500100 //1e6+100
#define MOD 1000000007
#define int long long
using namespace std;
int n;
int a[N],b[N];
int sa[N],sb[N],ss[N];
int s1,s2,s3,s4,s;
signed main(){
//ios::sync_with_stdio(false),cin.tie(0),cout.tie(0);
cin>>n;
rep(i,1,n) cin>>a[i];
rep(i,1,n) cin>>b[i];
rep(i,1,n){
a[i]+=a[i-1];
b[i]+=b[i-1];
a[i]%=MOD;
b[i]%=MOD;
sa[i]=a[i]+sa[i-1];
sb[i]=b[i]+sb[i-1];
sa[i]%=MOD;
sb[i]%=MOD;
ss[i]=((sa[i]*sb[i]%MOD)+ss[i-1])%MOD;
}
/*
计算x*y=(a[j]-a[i-1])*(b[j]-b[i-1])
=a[j]*b[j]-a[j]*b[i-1]-a[i-1]b[j]+a[i-1]b[i-1];
分开求每一个值
*/
rep(i,1,n){
s1=(ss[n]-ss[i-1]+MOD)%MOD;
s2=((sa[n]-sa[i-1]+MOD)%MOD * b[i-1])%MOD;
s3=((sb[n]-sb[i-1]+MOD)%MOD * a[i-1])%MOD;
s4=((n-i+1)*((a[i-1]*b[i-1])%MOD))%MOD;
s+=(s1-s2-s3+s4);
s%=MOD;
}
cout<<s;
return 0;
}
```
by lightme @ 2024-05-11 17:14:23
@[lightme](/user/940317) 我都一年没碰信竞了，去发新帖找别的大佬问吧
by wpy233 @ 2024-05-11 22:41:54