rt，感觉没什么问题啊

#include<bits/stdc++.h>

using namespace std;

const int mod = 1e9 + 7;
int n, x, ans, maxx, minn = 1e9;
map<int, int> mp;

int C(int k)
{
	return ((k * k - k) / 2) % mod;
}

int main()
{
	ios::sync_with_stdio(0);
	cin.tie(0), cout.tie(0);
	cin >> n;
	for(int i = 1; i <= n; i++)
	{
		cin >> x;
		maxx = max(x, maxx);
		minn = min(x, minn);
		mp[x]++;
	}
	for(int i = minn; i <= maxx; i++)
	{
		for(int j = i; j <= maxx; j++)
		{
			if(i != j)
			{
				ans += ((mp[i] * mp[j] % mod) * (C(mp[i + j]) % mod)) % mod;
			}
			else
			{
				ans += ((C(mp[i]) % mod) * (C(mp[i + j]) % mod)) %mod;
			}
		}
	}
	cout << ans;
	return 0;
}