POJ 2785 4 Values whose Sum is 0

#include <cstdio>
#include <algorithm>
#include <vector>
#define ll long long
using namespace std;

const int MAXN = 4000+5;
int a[MAXN], b[MAXN], c[MAXN], d[MAXN];
int cd[MAXN*MAXN];
int n;
pair<vector<int>::iterator, vector<int>::iterator> it;

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 c_d = -(a[i]+b[j]);
            it = equal_range(cd, cd+n*n, c_d);
            res += it.second-it.first;
        }
    }
    printf("%I64d\n", res);
}

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();
    return 0;
}