POJ 2002 Squares 哈希

题目链接： http://poj.org/problem?id=2002

#include <stdio.h>
#include <string.h>

const int prime = 999983;

struct Hash_table
{
    int x, y;
    struct Hash_table *next;
}*Hash[prime];

void Hash_insert(int x, int y)
{
    int key = (x + y) % prime;
    if(Hash[key] == NULL)
    {
        Hash[key] = new Hash_table;
        Hash[key]->x = x;
        Hash[key]->y = y;
        Hash[key]->next = NULL;
        return;
    }
    struct Hash_table *p = Hash[key];
    
    //必须注意！下面几行一定是p->next，如果改成：
    /*
    while(p != NULL)
        p = p->next;
    p = new Hash_table;
    p->x = x;
    p->y = y;
    p->next = NULL;
    */
    //是绝对不行的，因为这里调试了半天了。。。。。。
    //正解如下：
    while(p->next != NULL)
        p = p->next;
    p->next = new Hash_table;
    p->next->x = x;
    p->next->y = y;
    p->next->next = NULL;
}

bool Hash_search(int x, int y)
{
    int key = (x + y) % prime;
    struct Hash_table *p = Hash[key];
    while(p != NULL)
    {
        if(p->x == x && p->y == y)
            return 1;
        p = p->next;
    }
    return 0;
}

int main()
{
    int n, tx, ty, x[1010], y[1010];
    int x3, x4, y3, y4;
    while(scanf("%d", &n) != EOF && n)
    {
        memset(Hash, 0, sizeof(Hash));
        for(int i = 0; i < n; i++)
        {
            scanf("%d %d", &tx, &ty);
            x[i] = tx + 20000;
            y[i] = ty + 20000;
            Hash_insert(x[i], y[i]);
        }
        int ans = 0;
        for(int i = 0; i < n; i++)
        {
            for(int j = 0; j < n; j++)
            {
                if(j == i)continue;
                x3 = x[i]+(y[i]-y[j]);
                y3 = y[i]-(x[i]-x[j]);
                x4 = x[j]+(y[i]-y[j]);
                y4 = y[j]-(x[i]-x[j]);
                if(Hash_search(x3, y3) && Hash_search(x4, y4))
                    ans++;
            }
        }
        printf("%d
", ans/4);
    }
    return 0;
}