#include <bits/stdc++.h>
using namespace std;
#define int long long
const int N=1e6+5;
int a[N];
signed main(){
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	int n;
	cin>>n;
	for(int i=1;i<=n;i++){
		cin>>a[i];
	}
	int ans=0;
	sort(a+1,a+1+n);
	for(int i=1;i<=n;i++){
		map<int,int>cnt;
		for(int j=i+1;j<=n;j++){
			ans+=cnt[a[i]+a[j]];
			cnt[a[i]+a[j]]++;
		}
	}
	cout<<ans;
}

Thử xem ac ko bạn

#include <bits/stdc++.h
using namespace std;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);

    long long n, d = 0;
    cin >> n;
    vector<long long> a(n);
    for (long long i = 0; i < n; ++i) cin >> a[i];
    
    unordered_map<long long, long long> cnt;
    
    for (long long x1 = 1; x1 <= n - 3; ++x1) {
        for (long long x2 = x1 + 1; x2 <= n - 2; ++x2) {
            long long s1 = a[x1 - 1] + a[x2 - 1];
            cnt[s1]++;
        }
    }
    
    for (long long y1 = 3; y1 <= n - 1; ++y1) {
        for (long long y2 = y1 + 1; y2 <= n; ++y2) {
            long long s2 = a[y1 - 1] + a[y2 - 1];
            if (cnt.find(s2) != cnt.end()) {
                d += cnt[s2];
            }
        }
    }
    
    cout << d / 2;
    return 0;
}