Problem I: Inquiry I 1

Problem I: Inquiry I 1
I Inquiry I
The Bureau for Artificial Problems in Competitions wants you to solve the following problem:
Given n positive integers a1, . . . , an, what is the maximal value of

（a1^2+ a2^2 +…+ ak^2）*(ak+1+…+an)
Input
• A single line containing an integer 2 ≤ n ≤ 106
.
• Then follow n lines, the ith of which contains the integer 1 ≤ ai ≤ 100.
Output
Output the maximal value of the given expression.
Sample Input 1 Sample Output 1
5
2
1
4
3
5
168
Sample Input 2 Sample Output 2
2
1
1
1
Sample Input 3 Sample Output 3
10
8
5
10
9
1
4
12
6
3
13
10530

这道题比赛的时候我也是想了老半天，很晚才想出来，我们可以先将除了第n个当作后面的，其他全是前面的，然后将前面的一个弄到后面，以此类推，找到最大值。

#include<cstdio>
#include<iostream>
using namespace std;
typedef long long ll;
const int maxn=1e6+10;
int n,s[maxn];
ll p,q,sum;
int main(){
	cin>>n;
	for(int i=1;i<=n;i++){
		cin>>s[i];
		p+=s[i];
	}
	p-=s[1];q=s[1]*s[1];sum=p*q;
	for(int i=2;i<n;i++){
		p-=s[i];
		q+=s[i]*s[i];
		sum=max(sum,p*q);
	}
	printf("%lld\n",sum);
}