poj 1328 Radar Installation

http://poj.org/problem?id=1328

贪心：

题意：给定小岛的位置，问最小安装雷达数，雷达有一个半径，覆盖小岛。我竟然看错了题意，雷达都安装在x轴上，我竟然不知道。

分析：以小岛做一个圆，圆与x轴有2个交点a,b在区间[a,b]选点的问题，就转换区间选点问题，在小白书1的153页，先把区间b从小到大排序，取最后一个点。

                                                                                       Radar Installation

Time Limit: 1000MS		Memory Limit: 10000K
Total Submissions: 55095		Accepted: 12415

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

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<cmath>
using namespace std;
struct node
{
    double x,y;
}a[1005];
bool cmp(node d,node c)
{
    return d.y<c.y;
}
int main()
{
    int n,i,j,t=1,flag,ans;
    double d,r,w;
    while(~scanf("%d%lf",&n,&r))
    {
            ans=1;
           flag=0;
        if(n==0&&r==0)
             break;
        for(i=0;i<n;i++)
        {
            scanf("%lf%lf",&w,&d);
            if((r*r-d*d)<0)
                flag=1;
             a[i].x=w-sqrt(r*r-d*d);
              a[i].y=w+sqrt(r*r-d*d);
        }
          if(flag||r<=0)
           {
                printf("Case %d: -1\n",t++);
                continue;
           }
        sort(a,a+n,cmp);
        double temp=a[0].y;
        for(i=1;i<n;i++)
        {
            if(a[i].x>temp)
              {
                  ans++;
                  temp=a[i].y;
              }
        }
        printf("Case %d: %d\n",t++,ans);
    }
    return n;
}