poj2785 4 Values whose Sum is 0

思路：
折半枚举，二分。

实现：

#include <iostream>
#include <cstdio>
#include <algorithm>
using namespace std;
int a[4][4005];
int buf[16000005];
int main()
{
    int n;
    cin >> n;
    for (int i = 0; i < n; i++)
    {
        for (int j = 0; j < 4; j++)
        {
            cin >> a[j][i];
        }
    }
    int cnt = 0;
    for (int i = 0; i < n; i++)
    {
        for (int j = 0; j < n; j++)
        {
            buf[i * n + j] = a[0][i] + a[1][j];
        }
    }
    sort(buf, buf + n * n);
    for (int i = 0; i < n; i++)
    {
        for (int j = 0; j < n; j++)
        {
            int tmp = -a[2][i] - a[3][j];
            if (binary_search(buf, buf + n * n, tmp))
            {
                int l = lower_bound(buf, buf + n * n, tmp) - buf;
                int r = upper_bound(buf, buf + n * n, tmp) - buf;
                cnt += r - l;
            }
        }
    }
    cout << cnt << endl;
    return 0;
}