poj 2785 4 Values whose Sum is 0

http://poj.org/problem?id=2785

　　简单hash，不过不能用set，而且数组不能开太大。

　　才几十行的代码，当时个人赛居然不会... - -

#include <cstdlib>
#include <cstring>
#include <cstdio>

using namespace std;

const int maxn = 4001;
const int mod = 15999997;
int hash[maxn * maxn], cnt[maxn * maxn];
int rec[maxn][4];

void insert(int n){
    int p = (n << 2) % mod;

    if (p < 0) p += mod;
    while (cnt[p] && hash[p] != n) p++;
    hash[p] = n;
    cnt[p]++;
}

int count(int n){
    int p = (n << 2) % mod;

    if (p < 0) p += mod;
    while (cnt[p] && hash[p] != n) p++;
    return cnt[p];
}

int deal(int n){
    memset(cnt, 0, sizeof(cnt));
    for (int i = 0; i < n; i++){
        for (int j = 0; j < n; j++){
            insert(rec[i][0] + rec[j][1]);
        }
    }
    int ret = 0;

    for (int i = 0; i < n; i++){
        for (int j = 0; j < n; j++){
            ret += count(- rec[i][2] - rec[j][3]);
        }
    }

    return ret;
}

int main(){
    int n;

    while (~scanf("%d", &n)){
        for (int i = 0; i < n; i++){
            for (int j = 0; j < 4; j++){
                scanf("%d", &rec[i][j]);
            }
        }
        printf("%d\n", deal(n));
    }

    return 0;
}

——written by Lyon