样例OK，自制数据也没hack，但WA声一片

#include <iostream>
#include <algorithm>
using namespace std;
int n, s[5010], maxn;
unsigned long long ans;
const int MOD  = 1e9 + 7;
int main(){
    cin >> n;
    for(int i = 1; i <= n; i++){
        int tmp;
        cin >> tmp;
        s[tmp]++;
        maxn = max(maxn, tmp);
    }
    for(int i = 2; i <= maxn; i++){
        if(s[i] < 2) continue;
        for(int j = 1; j <= i / 2; j++){
            if(j != i - j && s[j] >= 1 && s[i - j] >= 1)
                ans += (s[j] * s[i - j]) % MOD;
            if(j == i - j && s[j] >= 2)
                ans += (s[j] * (s[j] - 1) / 2) % MOD;
            ans %= MOD;
        }
    }
    cout << ans;
    return 0;
}