Radar Installation
Time Limit : 2000/1000ms (Java/Other) Memory Limit : 20000/10000K (Java/Other)
Total Submission(s) : 54 Accepted Submission(s) : 28
Problem Description
Assume the coasting is an infinite straight line. Land is in one side of coasting, sea in the other. Each small island is a point locating in the sea side. And any radar installation, locating on the coasting, can only cover d distance, so an island in the sea can be covered by a radius installation, if the distance between them is at most d.
We use Cartesian coordinate system, defining the coasting is the x-axis. The sea side is above x-axis, and the land side below. Given the position of each island in the sea, and given the distance of the coverage of the radar installation, your task is to write a program to find the minimal number of radar installations to cover all the islands. Note that the position of an island is represented by its x-y coordinates. 
Figure A Sample Input of Radar Installations
Input
The input consists of several test cases. The first line of each case contains two integers n (1<=n<=1000) and d, where n is the number of islands in the sea and d is the distance of coverage of the radar installation. This is followed by n lines each containing two integers representing the coordinate of the position of each island. Then a blank line follows to separate the cases.
The input is terminated by a line containing pair of zeros
Output
For each test case output one line consisting of the test case number followed by the minimal number of radar installations needed. "-1" installation means no solution for that case.
Sample Input
3 2 1 2 -3 1 2 1 1 2 0 2 0 0
Sample Output
Case 1: 2 Case 2: 1
题解：先转化为区间点，再排序，区间找点；
代码：
#include<stdio.h>
#include<math.h>
#include<algorithm>
using namespace std;
struct Node{
double s,e;
};
int n,d,k;
Node area[];
int cmp(Node a,Node b){
return a.e<b.e;
}
int change(int x,int y){
if(y>d)return ;
double a,b,m=sqrt(d*d-y*y);
a=x-m;b=x+m;
area[k].s=a;area[k].e=b;k++;
return ;
}
int main(){int t,x,y,flot,temp,num,l=;
while(scanf("%d%d",&n,&d),n||d){k=;flot=;temp=;num=;l++;
for(int i=;i<n;i++){
scanf("%d%d",&x,&y);
t=change(x,y);
if(!t)flot=;
}
sort(area,area+k,cmp);
for(int i=;i<k;i++){
if(area[i].s>area[temp].e)temp=i,num++;
}
printf("Case %d: %d\n",l,num);
}
return ;
}