poj_1328_Radar Installation(贪心)
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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 11 2
0 20 0
Sample Output
Case 1: 2
Case 2: 1
思路
题意:用最少的雷达覆盖所有的岛屿。
本来想从雷达入手去做,可是好多东西是变的,没法解决,也比较麻烦。上网找了题解之后是从岛屿入手。因为雷达都安在一条线上,只有x的变化,而且雷达的覆盖范围一定,这就转化成了类似区间覆盖问题。
雷达覆盖半径以确定,通过岛屿(x,y)位置可以找出最大区间,(以雷达覆盖半径为半径,岛屿为圆心画圆,即可得到区间)将区间的左端点从小到大排序,判断岛屿是否在范围内即可。
如果下一个区间的左端点与上一个区间的右端点不重合,则雷达数+1,并更新右端点,如果下一个区间完全被上一个包含在内,则更新右端点。
用结构体:区间左右端点
sort排序函数:按左端点从小到大,若左端点相等,则按右端点从小到大
代码
#include <iostream>#include<algorithm>#include<math.h>using namespace std;struct zuobiao{ double x; //区间左 double y; //区间右};bool cmp(zuobiao a , zuobiao b ){ if(a.x!=b.x) return a.x<b.x; return a.y<b.y;}int main(){ struct zuobiao s[1005]; int n,d,count,num=1; int i,x,y; double t,p; while(cin>>n>>d) { count=1; if(n==0&&d==0) break; for(i=0;i<n;i++) { cin>>x>>y; if(y>d) count=-1; t=sqrt(d*d-y*y);//解直角三角形 s[i].x=x-t;//区间左端点 s[i].y=x+t;//区间右端点 } if(count!=-1) { sort(s,s+n,cmp); p=s[0].y;//区间右端点 for(i=1;i<n;i++) { if(s[i].x>p)//下一个区间左端点与上一个区间右端点没有重合 { count++; p=s[i].y;//更新右端点 } else if(s[i].y<p)//下一个区间完全被上一个包含在内 p=s[i].y; } } cout<<"Case "<<num<<": "<<count<<endl; num++; } return 0;}
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