（哈希）Squares (p2002)

这个要注重二分的查找，，。先是二分查找的方法，

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<queue>
#include<vector>
#include<cmath>
#include<set>
#include<cstring>

using namespace std;

#define N 1001

struct my
{
int x,y;

bool operator<(my b)
{
if (b.x!=x)
return x<b.x;
else
return y<b.y;
}

bool operator==(my b)
{
if (b.x==x&&b.y==y)
return true;
else
return false;
}

void put()
{
cout<<x<<' '<<y<<"    ";
}

}
p
;
int n;

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

bool find (my a)
{
int i,j,k;

int s=0;
int m;
int e=n-1;
while (s<=e)
{
m=(e+s)/2;

if (p[m]==a)
{
return true;
}
else if (p[m].x>a.x||(p[m].x==a.x&p[m].y>a.y))
{
e=m-1;
}
else
s=m+1;

}
return false;
}

int main()
{
freopen("in.txt","r",stdin);
int i,j,k;

while (cin>>n,n)
{
for (i=0;i<n;i++)
{
cin>>p[i].x>>p[i].y;
}
sort(p,p+n);
int ans=0;
my a,b;
for (i=0;i<n;i++)
{
for (j=i+1;j<n;j++)
{
a.x=p[i].y-p[j].y+p[i].x;
a.y=p[j].x-p[i].x+p[i].y;
b.x=p[i].y-p[j].y+p[j].x;
b.y=p[j].x-p[i].x+p[j].y;

if (find(a)&&find(b))
ans++;
}
}
cout<<ans/2<<endl;
}
return 0;
}

Squares

Time Limit: 3500MS		Memory Limit: 65536K
Total Submissions: 11803		Accepted: 4319

Description
A square is a 4-sided polygon whose sides have equal length and adjacent sides form 90-degree angles. It is also a polygon such that rotating about its centre by 90 degrees gives the same polygon. It is not the only polygon with
the latter property, however, as a regular octagon also has this property.

So we all know what a square looks like, but can we find all possible squares that can be formed from a set of stars in a night sky? To make the problem easier, we will assume that the night sky is a 2-dimensional plane, and each star is specified by its x
and y coordinates.

Input
The input consists of a number of test cases. Each test case starts with the integer n (1 <= n <= 1000) indicating the number of points to follow. Each of the next n lines specify the x and y coordinates (two integers) of each
point. You may assume that the points are distinct and the magnitudes of the coordinates are less than 20000. The input is terminated when n = 0.
Output
For each test case, print on a line the number of squares one can form from the given stars.
Sample Input

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

Sample Output

1
6
1