描述
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.
- 输入
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.You can assume the number of test cases is less than 20 - 输出
For each test case, print on a line the number of squares one can form from the given stars. - 样例输入
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 - 样例输出
1
6
1
分析:
题目的意思其实很简单,就是对于给出的一系列二维坐标系中的点,其中的四个点构成一个正方形,这样不同的正方形一共有多少个。
代码:
#include<stdio.h>
#include<iostream>
#include<algorithm>
using namespace std;
int n;
struct Node
{
int x;
int y;
} node[1009];
bool cmp(Node a,Node b)///这些点如果横坐标相同的话,就按照纵坐标从小到大排序;否则就直接按照横坐标从小到大排序
{
if(a.x==b.x)
return a.y<b.y;
else
return a.x<b.x;
}
bool Search(int x,int y)///采用二分查找法,查一下求出的点是否在已有点的序列中
{
int left=0;
int right=n;
int mid;
while(left<=right)
{
mid=(left+right)/2;
if(node[mid].x==x&&node[mid].y==y)
return true;
else if(node[mid].x==x&&node[mid].y<y||node[mid].x<x)///递归在右区间中找
left=mid+1;
else///递归在左区间中找
right=mid-1;
}
return false;
}
int main()
{
while(~scanf("%d",&n)&&n)
{
int ans=0;
for(int i=0; i<n; i++)
scanf("%d%d",&node[i].x,&node[i].y);
sort(node,node+n,cmp);
int x1,y1,x2,y2;
for(int i=0; i<n; i++)
for(int j=i+1; j<n; j++)
{
///这样找的话相当于找的是这条线上方或左方的图形,不理解的话自己用坐标试试
x1=node[j].x-(node[j].y-node[i].y);
y1=node[j].y+(node[j].x-node[i].x);
if(!Search(x1,y1)) continue;
x2=node[i].x-(node[j].y-node[i].y);
y2=node[i].y+(node[j].x-node[i].x);
if(!Search(x2,y2)) continue;
ans++;
}
printf("%d
",ans/2);///每一个图形都可以由两条边找到,下面和右面的边
}
return 0;
}