4 Values whose Sum is 0 POJ - 2785

#include<cstdio>
#include<iostream>
#include<algorithm>
#include<cstring>
#include<cmath>
#include<cstdlib>
#include<queue>
#include<set>
#include<vector>
using namespace std;
#define INF 0x3f3f3f3f
#define eps 1e-10
typedef long long ll;
const int maxn = 4002;
const int mod = 1e9 + 7;
int gcd(int a, int b) {
    if (b == 0) return a;  return gcd(b, a % b);
}

int n;
int a[maxn],b[maxn],c[maxn],d[maxn];
int cd[maxn*maxn];

void solve()
{
    for(int i=0;i<n;i++)
        for(int j=0;j<n;j++)
        {
            cd[i*n+j]=c[i]+d[j];
        }
    sort(cd,cd+n*n);
    ll res=0;
    for(int i=0;i<n;i++)
        for(int j=0;j<n;j++)
        {
            int ab=-(a[i]+b[j]);
            res+=upper_bound(cd,cd+n*n,ab)-lower_bound(cd,cd+n*n,ab);
        }
    cout<<res<<endl;
}
int main()
{
    scanf("%d",&n);
    for(int i=0;i<n;i++)
        scanf("%d %d %d %d",&a[i],&b[i],&c[i],&d[i]);
    
    solve();
}