2 solutions

  • 1
    @ 2026-3-15 14:06:19
    #include<bits/stdc++.h>
    typedef long long LL;
    using namespace std;
    const int P=1000000007;
    int main()
    {
    	int n;
    	cin>>n;
    	vector<int> a(n);
    	LL sum=0;
    	for(auto &x:a)
    	{
    		cin>>x;
    		sum+=x; //累积求和
    		sum%=P; //只保留余数
    	}
    	LL ans=0;
    	for(int i=0;i<n;i++)
    	{
    		sum-=a[i]; //更新后面的乘积
    		if(sum<0) sum+=P; //正余数
    		ans+=a[i]*sum;
    		ans%=P;
    	}
    	cout<<ans;
    	return 0;
    }
    

    [ABC177C] 数对乘积之和(Sum of product of pairs)

    Information

    ID
    2712
    Time
    1000ms
    Memory
    256MiB
    Difficulty
    5
    Tags
    # Submissions
    15
    Accepted
    6
    Uploaded By