poj 2785 4 Values whose Sum is 0

#include<iostream>
#include<cstdio>
#include<algorithm>
using namespace std;

int a[4003];
int b[4003];
int c[4003];
int d[4003];
int e[16000003];
int main()
{
	int n;
	cin>>n;
	int i;
	int j;
	for(i=0;i<n;i++)
		cin>>a[i]>>b[i]>>c[i]>>d[i];
	for(i=0;i<n;i++)
	{
		for(j=0;j<n;j++)
			e[i*n+j]=a[i]+b[j];	
	}
	sort(e,e+n*n);
	int   res;
	int sum=0;
	for(i=0;i<n;i++)
		for(j=0;j<n;j++)
		{
			res=c[i]+d[j];
			sum+=upper_bound(e,e+n*n,-res)-lower_bound(e,e+n*n,-res);//返回个数，中间有几个相同的值也包含了，好巧妙
		}
	cout<<sum<<endl;
}