POJ-2031-Building a Space Station

Building a Space Station

Time Limit: 1000MS Memory Limit: 30000KTotal Submissions: 8178 Accepted: 3927

Description

You are a member of the space station engineering team, and are assigned a task in the construction process of the station. You are expected to write a computer program to complete the task.
The space station is made up with a number of units, called cells. All cells are sphere-shaped, but their sizes are not necessarily uniform. Each cell is fixed at its predetermined position shortly after the station is successfully put into its orbit. It is quite strange that two cells may be touching each other, or even may be overlapping. In an extreme case, a cell may be totally enclosing another one. I do not know how such arrangements are possible.

All the cells must be connected, since crew members should be able to walk from any cell to any other cell. They can walk from a cell A to another cell B, if, (1) A and B are touching each other or overlapping, (2) A and B are connected by a `corridor', or (3) there is a cell C such that walking from A to C, and also from B to C are both possible. Note that the condition (3) should be interpreted transitively.

You are expected to design a configuration, namely, which pairs of cells are to be connected with corridors. There is some freedom in the corridor configuration. For example, if there are three cells A, B and C, not touching nor overlapping each other, at least three plans are possible in order to connect all three cells. The first is to build corridors A-B and A-C, the second B-C and B-A, the third C-A and C-B. The cost of building a corridor is proportional to its length. Therefore, you should choose a plan with the shortest total length of the corridors.

You can ignore the width of a corridor. A corridor is built between points on two cells' surfaces. It can be made arbitrarily long, but of course the shortest one is chosen. Even if two corridors A-B and C-D intersect in space, they are not considered to form a connection path between (for example) A and C. In other words, you may consider that two corridors never intersect.

Input

The input consists of multiple data sets. Each data set is given in the following format.

n
x1 y1 z1 r1
x2 y2 z2 r2
...
xn yn zn rn

The first line of a data set contains an integer n, which is the number of cells. n is positive, and does not exceed 100.

The following n lines are descriptions of cells. Four values in a line are x-, y- and z-coordinates of the center, and radius (called r in the rest of the problem) of the sphere, in this order. Each value is given by a decimal fraction, with 3 digits after the decimal point. Values are separated by a space character.

Each of x, y, z and r is positive and is less than 100.0.

The end of the input is indicated by a line containing a zero.

Output

For each data set, the shortest total length of the corridors should be printed, each in a separate line. The printed values should have 3 digits after the decimal point. They may not have an error greater than 0.001.

Note that if no corridors are necessary, that is, if all the cells are connected without corridors, the shortest total length of the corridors is 0.000.

Sample Input

310.000 10.000 50.000 10.00040.000 10.000 50.000 10.00040.000 40.000 50.000 10.000230.000 30.000 30.000 20.00040.000 40.000 40.000 20.00055.729 15.143 3.996 25.8376.013 14.372 4.818 10.67180.115 63.292 84.477 15.12064.095 80.924 70.029 14.88139.472 85.116 71.369 5.5530

Sample Output

20.0000.00073.834

Source

Japan 2003 Domestic

题意：

该题说的是在太空中有n个基本单位，现在有它们的空间坐标x,y,z和半径r;现在让你求要让各个基本单位连结起来需要添加的长廊最短要多少（两个基本单位只要有接触，或则重叠部分就视为连结）。

可以看出来这是一个类似于三维的最小生成树，就是在求两点之间的距离时，如果两基本单位连结，那么它们的距离就为0。

求两基本的距离的公式为：double Dis = sqrt((a[u].x-a[v].x)*(a[u].x-a[v].x) + (a[u].y-a[v].y)*(a[u].y-a[v].y) + (a[u].z-a[v].z)*(a[u].z-a[v].z))-a[u].r-a[v].r;

其余操作和简单的最小生成树的操作是一样的。

#include <cstdio>#include <iostream>#include <cstring>#include <cstdlib>#include <algorithm>#include <cmath>const int Maxn = 110;#define INF 0x3f3f3f3fusing namespace std;struct node{double x,y,z,r;}a[Maxn];int n;double Map[Maxn][Maxn],vist[Maxn],dis[Maxn];double distance(int u,int v){double Dis = sqrt((a[u].x-a[v].x)*(a[u].x-a[v].x) + (a[u].y-a[v].y)*(a[u].y-a[v].y) + (a[u].z-a[v].z)*(a[u].z-a[v].z))-a[u].r-a[v].r;if(Dis < 0)return 0;return Dis;}void prim(){memset(vist,0,sizeof(vist));for(int i = 0; i < n; i++)dis[i] = Map[0][i];vist[0] = 1;int pos;double Min,sum = 0;for(int i = 1; i < n; i++){Min = INF;for(int j = 0; j < n; j++)if(!vist[j] && Min > dis[j]){Min = dis[j];pos = j;}sum += Min;vist[pos] = 1;for(int j = 0; j < n; j++)if(!vist[j] && dis[j] > Map[pos][j])dis[j] = Map[pos][j];}printf("%.3lf\n",sum);}int main(){while(~scanf("%d",&n) && n){memset(Map,0,sizeof(Map));for(int i = 0; i < n; i++)scanf("%lf %lf %lf %lf",&a[i].x,&a[i].y,&a[i].z,&a[i].r);for(int i = 0; i < n; i++)for(int j = 0; j < n; j++)if(i == j)Map[i][j] = 0;elseMap[i][j] = distance(i,j);prim();}return 0;}