POJ2785 4 Values whose Sum is 0

求出前两列的两两一对的和存到数组里，然后枚举后面两行两两一对的和并求他们的相反数，然后在对数组进行排序，使用二分查找后面两行两两一对的和的相反数。

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
int n;
const int maxn = 4010;
int arr[maxn][4];
vector<int>temp;
int main()
{
    cin >> n;
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < 4; j++) {
            cin >> arr[i][j];
        }
    }
    int ans = 0;
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {
            temp.push_back(arr[i][0] + arr[j][1]);
        }
    }
    sort(temp.begin(), temp.end());
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {
            int num = -(arr[i][2] + arr[j][3]);
            ans+= upper_bound(temp.begin(), temp.end(), num) - lower_bound(temp.begin(), temp.end(), num);
        }
    }

    cout << ans;
}