poj2785

大空间大时间的题，把4个数字分成两份，分别两两求和，得到两个长度n*n的一维数组，排序后比较进行匹配即可。

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

#define maxn 4004

int n;
int f1[maxn * maxn];
int f2[maxn * maxn];
int a[maxn], b[maxn], c[maxn], d[maxn];

int main()
{
    //freopen("t.txt", "r", stdin);
    scanf("%d", &n);
    for (int i = 0; i < n; i++)
        scanf("%d%d%d%d", &a[i], &b[i], &c[i], &d[i]);
    for (int i = 0; i < n; i++)
        for (int j = 0; j < n; j++)
            f1[i * n + j] = a[i] + b[j];
    for (int i = 0; i < n; i++)
        for (int j = 0; j < n; j++)
            f2[i * n + j] = c[i] + d[j];
    sort(f1, f1 + n * n);
    sort(f2, f2 + n * n);
    int r = n * n - 1;
    int ans = 0;
    for (int i = 0; i < n * n; i++)
    {
        while (r >= 0 && f1[i] + f2[r] > 0)
            r--;
        if (r < 0)
            break;
        int temp = r;
        while (temp >= 0 && f1[i] + f2[temp] == 0)
            ans++, temp--;
    }
    printf("%d\n", ans);
    return 0;
}