POJ 2002

纯二分 水

#include <cstdio>
#include <cstring>
#include <algorithm>

using std::memset;
using std::sort;

const int MAX = 2005;
int n;

struct position
{
	int x,y;
}star[MAX];;

bool cmp(position a,position b)
{
	if(a.x == b.x) return a.y < b.y;
	return a.x < b.x;
}

int cmp2(position a, position b)
{
	if(a.x == b.x)
	{
		if(a.y == b.y)
			return 0;
		else if(a.y < b.y)
			return -1;
		else
			return 1;
	}
	else if(a.x < b.x)
		return -1;
	return 1;
}

bool binsearch(int l, int r, position x)
{
	while(l+1 < r)
	{
		int m = (l+r)>>1;
		if(cmp2(x, star[m]) == 0)
			return true;
		else if(cmp2(x, star[m]) == -1)
			r = m;
		else
			l = m;
	}
	return false;
}

int main()
{
	while(scanf("%d", &n) && n)
	{
		for(int i = 0 ; i < n; i++)
		{
			scanf("%d%d", &star[i].x, &star[i].y);
		}

		sort(star, star+n, cmp);

		//for(int i = 0 ; i < n; i++)
		//{
		//	printf("%d%d\n", star[i].x, star[i].y);
		//}
		int count = 0;

		for(int i = 0 ; i < n; i++)
		{
			for(int j = i+1; j < n; j++)
			{
				int dx = star[j].x - star[i].x;
				int dy = star[j].y - star[i].y;

				position temp;
				
				temp.x = star[i].x + dy;
				temp.y = star[i].y - dx;

				if(binsearch(-1, n, temp) == false) continue;

				temp.x = star[j].x + dy;
				temp.y = star[j].y - dx;

				if(binsearch(-1, n, temp) == false) continue;

				count++;
			}
		}

		printf("%d\n", count/2);
	}
}