K - Parallelogram Counting解题报告（来自网络）

K - Parallelogram Counting

Time Limit:5000MS     Memory Limit:65536KB     64bit IO Format:%I64d & %I64u

Submit  Status  Practice  POJ 1971

Description

There are n distinct points in the plane, given by their integer coordinates. Find the number of parallelograms whose vertices lie on these points. In other words, find the number of 4-element subsets of these points that can be written as {A, B, C, D} such that AB || CD, and BC || AD. No four points are in a straight line.

Input

The first line of the input contains a single integer t (1 <= t <= 10), the number of test cases. It is followed by the input data for each test case. 
The first line of each test case contains an integer n (1 <= n <= 1000). Each of the next n lines, contains 2 space-separated integers x and y (the coordinates of a point) with magnitude (absolute value) of no more than 1000000000. 

Output

Output should contain t lines. 
Line i contains an integer showing the number of the parallelograms as described above for test case i. 

Sample Input

2
6
0 0
2 0
4 0
1 1
3 1
5 1
7
-2 -1
8 9
5 7
1 1
4 8
2 0
9 8

Sample Output

5
6

#include<memory.h>
#include<stdlib.h>
#include<stdio.h>
#define MAX 1000
#define HMAX  0x3fffff
#define P 0x1fffff
//将每个中点的x，y都记录下来，如果
//同时记录拥有这个中点的线段的个数
struct Point{
	int x;
	int y;
	int count;
	int next;
};
Point p[MAX*MAX];
int head[HMAX];
int x[MAX],y[MAX];
int main()
{
	int t,n;
	int zz,xx,yy,xtmp,ytmp,ptr,res,h;
	bool flag;
	scanf("%d",&t);
	while(t--)
	{
		ptr = 1;
		res = 0;
		memset(head,0,sizeof(head));
		memset(p,0,sizeof(p));
		scanf("%d",&n);
		for(int i = 0; i < n; i++)
		{
			scanf("%d%d",&xtmp,&ytmp);
			for(int j = 0; j < i; j++)
			{
				flag = true;
				xx = xtmp + x[j];
				yy = ytmp + y[j];
				//h = xx & P + yy & P; --用这种方法反而更慢...
				//哈希中点坐标之和
				h = abs(xx + yy)%HMAX;
				zz = head[h];
				while(zz){
					if(p[zz].x == xx && p[zz].y == yy){
						p[zz].count++;
						res+=p[zz].count;
						flag = false;
						break;
					}
					else
						zz = p[zz].next;
				}
				//插入新的中点
				if(flag)	{
					p[ptr].x = xx;
					p[ptr].y = yy;
					p[ptr].next = head[h];
					head[h] = ptr++;
				}
			}
			x[i] = xtmp;
			y[i] = ytmp;
		}
		printf("%d/n",res);
	}
	return 0;
}