poj 2785 4 Values whose Sum is 0

#include <cstdio>
#include <cmath>
#include <cctype>
#include <iostream>
#include <cstring>
#include <algorithm>
#include <string>
#include <stack>
#include <vector>
#include <map>
#include <set>
using namespace std;
typedef long long LL;

string s;
LL a[4005],b[4005],c[4005],d[4005];
LL cd[16000001] = {0};
LL n,res = 0;
int main() {
  // freopen("test.in","r",stdin);
  ios::sync_with_stdio(false);
  cin >> n;
  for (int i=0;i<n;i++){
    cin >> a[i] >> b[i] >> c[i] >> d[i];
  }
  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);
  for (int i=0;i<n;i++){
    for (int j=0;j<n;j++){
      LL now = a[i] + b[j];
      LL need = 0 - now;
      res += upper_bound(cd,cd+n*n,need) - lower_bound(cd,cd+n*n,need);
    }
  }
  cout << res;
  return 0;
}