lightoj 1058 - Parallelogram Counting (几何，平行四边形)

1058 - Parallelogram Counting

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Time Limit: 2 second(s)

Memory Limit: 32 MB

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

Input starts with an integer T (≤ 15), denoting the number of test cases.

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 than1000000000.

Output

For each case, print the case number and the number of parallelograms that can be formed.

Sample Input	Output for 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	Case 1: 5 Case 2: 6

 

给n点看有多少个平行四边形

定理：中点相同构成一个平行四边形

代码：

#include
#include
using namespace std;

struct edge
{
	int x,y;
}a[1010],mid[1000010];
bool cmp(edge a,edge b)
{
	if(a.x==b.x)
		return a.y