poj 2349 Arctic Network 最小生成树

Description

The Department of National Defence (DND) wishes to connect several northern outposts by a wireless network. Two different communication technologies are to be used in establishing the network: every outpost will have a radio transceiver and some outposts will in addition have a satellite channel. 
Any two outposts with a satellite channel can communicate via the satellite, regardless of their location. Otherwise, two outposts can communicate by radio only if the distance between them does not exceed D, which depends of the power of the transceivers. Higher power yields higher D but costs more. Due to purchasing and maintenance considerations, the transceivers at the outposts must be identical; that is, the value of D is the same for every pair of outposts. 

Your job is to determine the minimum D required for the transceivers. There must be at least one communication path (direct or indirect) between every pair of outposts.

Input

The first line of input contains N, the number of test cases. The first line of each test case contains 1 <= S <= 100, the number of satellite channels, and S < P <= 500, the number of outposts. P lines follow, giving the (x,y) coordinates of each outpost in km (coordinates are integers between 0 and 10,000).

Output

For each case, output should consist of a single line giving the minimum D required to connect the network. Output should be specified to 2 decimal points.

Sample Input

12 40 1000 3000 600150 750

Sample Output

212.13

题意及分析：

有p个哨所，在它们之间建立通信网络，使得两两之间可以相互联系。有两种方式，一种是卫星，一种是无线电。无线电需要发射装置，卫星可以直接联系。卫星与非卫星点之间依旧需要发射装置。给出了各点的坐标和卫星的数量s。因为要求所有的发射装置功率相同，所以要求功率的最大值。功率的大小跟距离相关。

用最小生成树解释就是，先构造一棵最小生成树。在一棵最小生成树里，各点是联通的，并且每个点到树的距离最小。在这个前提下，排序之后，然后最大的几条边去掉（也就是相应的点 使用卫星 联系），剩下的边里最大的值就是要求的。

AC代码：