Problem:
Building a Space Station

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

3
10.000 10.000 50.000 10.000
40.000 10.000 50.000 10.000
40.000 40.000 50.000 10.000
2
30.000 30.000 30.000 20.000
40.000 40.000 40.000 20.000
5
5.729 15.143 3.996 25.837
6.013 14.372 4.818 10.671
80.115 63.292 84.477 15.120
64.095 80.924 70.029 14.881
39.472 85.116 71.369 5.553
0

Output for the Sample Input

20.000
0.000
73.834

Solution in JAVA (judge verdict: Accepted)

import java.io.*;
import java.util.*;

class Main{

    void solve(){        
        int N = ni();
        List<Double> ans = new ArrayList<>();

        while(N != 0){
            double[] x = new double[N];
            double[] y = new double[N];
            double[] z = new double[N];
            double[] r = new double[N];
            UnionFind uf = new UnionFind(N);
            List<Edge> edges = new ArrayList<>();
            for(int i = 0; i < N; i++){
                x[i] = scan.nextDouble();
                y[i] = scan.nextDouble();
                z[i] = scan.nextDouble();
                r[i] = scan.nextDouble();
            }
            for(int i = 0; i < N - 1; i++){
                for(int j = i + 1; j < N; j++){
                    double dist = Math.sqrt((x[i] - x[j]) * (x[i] - x[j]) + (y[i] - y[j]) * (y[i] - y[j]) + (z[i] - z[j]) * (z[i] - z[j]));
                    if(r[i] + r[j] >= dist){
                        uf.unite(i, j);
                    } else {
                        Edge edge = new Edge(i, j, dist - r[i] - r[j]);
                        edges.add(edge);
                    }
                }
            }
            Collections.sort(edges, new EdgeComparator());
    
            double sum = 0.0;
            for(int i = 0; i < edges.size(); i++){ 
                Edge edge = edges.get(i);
                if(uf.root(edge.from) == uf.root(edge.to)) continue;
                sum += edge.cost;
                uf.unite(edge.from, edge.to);
            }
    
            ans.add(sum);
            N = ni();
        }
        
        for(double val : ans){
            System.out.printf("%.3f\n", val);
        }

    }

    public static void main(String[] args){
        new Main().solve();
    }

    private FastScanner scan = new FastScanner();
    private final int MOD = 1_000_000_007;
    private final int INF = 2147483647;
    private final long LINF = 9223372036854775807L;
    private long[] fac;
    private long[] finv;
    private long[] inv;

    // Scanner
    int ni(){ return scan.nextInt();}
    int[] ni(int n){int[] a = new int[n]; for(int i = 0; i < n; i++){a[i] = ni();} return a;}
    int[][] ni(int y, int x){int[][] a = new int[y][x];
        for(int i = 0; i < y; i++){for(int j = 0; j < x; j++){a[i][j] = ni();}} return a;}
    long nl(){return scan.nextLong();}
    long[] nl(int n){long[] a = new long[n]; for(int i = 0; i < n; i++){a[i] = nl();} return a;}
    long[][] nl(int y, int x){long[][] a = new long[y][x];
        for(int i = 0; i < y; i++){for(int j = 0; j < x; j++){a[i][j] = nl();}} return a;}
    String ns(){return scan.next();}
    String[] ns(int n){String[] a = new String[n]; for(int i = 0; i < n; i++){a[i] = ns();} return a;}
    String[][] ns(int y, int x){String[][] a = new String[y][x];
        for(int i = 0; i < y; i++){for(int j = 0; j < x; j++){a[i][j] = ns();}} return a;}

    // Mathmatics
    int max(int a, int b){return Math.max(a, b);}
    long max(long a, long b){return Math.max(a, b);}
    double max(double a, double b){return Math.max(a, b);}
    int max(int[] a){int max = a[0]; for(int value:a){max = max(max,value);} return max;}
    long max(long[] a){long max = a[0]; for(long value:a){max = max(max,value);} return max;}
    double max(double[] a){double max = a[0]; for(double value:a){max = max(max,value);} return max;}
    int min(int a, int b){return Math.min(a, b);}
    long min(long a, long b){return Math.min(a, b);}
    double min(double a, double b){return Math.min(a, b);}
    int min(int[] a){int min = a[0]; for(int value:a){min = min(min,value);} return min;}
    long min(long[] a){long min = a[0]; for(long value:a){min = min(min,value);} return min;}
    double min(double[] a){double min = a[0]; for(double value:a){min = min(min,value);} return min;}
    long sum(int[] a){long sum = 0; for(int value:a){sum += value;} return sum;}
    long sum(long[] a){long sum = 0; for(long value:a){sum += value;} return sum;}
    double sum(double[] a){double sum = 0; for(double value:a){sum += value;} return sum;}
    int gcd(int a, int b){return b == 0 ? a : gcd(b, a % b);}
    long gcd(long a, long b){return b == 0 ? a : gcd(b, a % b);}
    int lcm(int a, int b){return a / gcd(a, b) * b;}
    long lcm(long a, long b){return a / gcd(a, b) * b;}
    long fact(int n){ if(n == 0){ return 1; } long a = n; for(long i = n - 1; i >= 2; i--){ a = a % MOD * i; } return a; }
    long fact(long n){ if(n == 0){ return 1; } long a = n; for(long i = n - 1; i >= 2; i--){ a = a % MOD * i; } return a; }
    // nPr(int)
    long npr(int n, int r){
        long a = 1;
        for(int i = n; i > n - r; i--){
            a *= i;
        }
        return a;
    }
    // nPr(long)
    long npr(long n, long r){
        long a = 1;
        for(long i = n; i > n - r; i--){
            a *= i;
        }
        return a;
    }
    // 素数判定(int)
    boolean checkPrime(int n){
        for(int i = 2; i * i <= n; i++){
            if(n % i == 0){
                return false;
            }
        }
        return true;
    }
    // 素数判定(long)
    boolean checkPrime(long n){
        for(long i = 2; i * i <= n; i++){
            if(n % i == 0){
                return false;
            }
        }
        return true;
    }
    // mod pにおける累乗 a^n
    long modpow(long a, long n, long p){
        long res = 1;
        while(n > 0){
            if((n & 1) == 1){
                res = res * a % p;
            }
            a = a * a % p;
            n = n >> 1;
        }
        return res;
    }
    // mod pにおけるaの逆元a^-1
    long modinv(long a, long p){
        return modpow(a, p - 2, p);
    }
    // fac,finv,invの初期化
    void comInit(int max){
        fac = new long[max];
        finv = new long[max];
        inv = new long[max];
        fac[0] = fac[1] = 1;
        finv[0] = finv[1] = 1;
        inv[1] = 1;
        for(int i = 2; i < max; i++){
            fac[i] = fac[i - 1] * i % MOD;
            inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
            finv[i] = finv[i - 1] * inv[i] % MOD;
        }
    }
    // 二項係数nCr
    long com(int n, int r){
        if(n < r || (n < 0 || r < 0)){
            return 0;
        }
        return fac[n] * (finv[r] * finv[n - r] % MOD) % MOD;
    }
    // 二項係数nCr（nが10^9など巨大なとき用）
    long ncr(long n, long k){
        long a = 1;
        long b = 1;
        for(int i = 1; i <= k; i++){
            a = a * (n + 1 - i) % MOD;
            b = b * i % MOD;
        }
        
        return modinv(b, MOD) * a % MOD;
    }


    // Array
    void sort(int[] a){ Arrays.sort(a);}
    void sort(long[] a){ Arrays.sort(a);}
    void sort(double[] a){ Arrays.sort(a);}
    void sort(String[] a){ Arrays.sort(a);}
    int[] reverse(int[] a){
        int[] reversed = new int[a.length];
        for(int i = 0; i < a.length; i++){
            reversed[a.length - i - 1] = a[i];
        }
        return reversed;
    }
    long[] reverse(long[] a){
        long[] reversed = new long[a.length];
        for(int i = 0; i < a.length; i++){
            reversed[a.length - i - 1] = a[i];
        }
        return reversed;
    }
    double[] reverse(double[] a){
        double[] reversed = new double[a.length];
        for(int i = 0; i < a.length; i++){
            reversed[a.length - i - 1] = a[i];
        }
        return reversed;
    }
    char[] reverse(char[] a){
        char[] reversed = new char[a.length];
        for(int i = 0; i < a.length; i++){
            reversed[a.length - i - 1] = a[i];
        }
        return reversed;
    }
    String[] reverse(String[] a){
        String[] reversed = new String[a.length];
        for(int i = 0; i < a.length; i++){
            reversed[a.length - i - 1] = a[i];
        }
        return reversed;
    }
    boolean[] reverse(boolean[] a){
        boolean[] reversed = new boolean[a.length];
        for(int i = 0; i < a.length; i++){
            reversed[a.length - i - 1] = a[i];
        }
        return reversed;
    }
    void fill(int[] array, int x) { Arrays.fill(array, x); }
    void fill(long[] array, long x) { Arrays.fill(array, x); }
    void fill(double[] array, double x) { Arrays.fill(array, x); }
    void fill(boolean[] array, boolean x) { Arrays.fill(array, x); }
    void fill(int[][] array, int x) { for(int a[] : array) { fill(a, x); } }
    void fill(long[][] array, long x) { for(long a[] : array) { fill(a, x); } }
    void fill(double[][] array, double x) { for(double a[] : array) { fill(a, x); } }
    void fill(boolean[][] array, boolean x) { for(boolean a[] : array) { fill(a, x); } }
    void fill(int[][][] array, int x) { for(int[][] ary : array) { for(int[] a : ary){ fill(a, x); } } }
    void fill(long[][][] array, long x) { for(long[][] ary : array) { for(long[] a : ary){ fill(a, x); } } }

    // Algorithm
    // 深さ優先探索
    /*
        HashMap<Integer, List<Integer>> graph = new HashMap<>();
        List<Integer> list;
    */
    int minCost = INF;
    void dfs(HashMap<Integer, List<Integer>> graph, int v, boolean[] seen, int[] cost, PriorityQueue<Integer> queue){
        seen[v] = true;
        queue.add(cost[v]);
        if(!graph.containsKey(v) || graph.get(v).size() == 0) return;
        List<Integer> todo = graph.get(v);
        for(int next : todo){
            if(seen[next]) continue;
            dfs(graph, next, seen, cost, queue);
        }
    }
    // 幅優先探索
    /*
        HashMap<Integer, List<Integer>> graph = new HashMap<>();
        List<Integer> list;
    */
    void bfs(HashMap<Integer, List<Integer>> graph, int v, int[] dist){
        Queue<Integer> que = new ArrayDeque<>();
        que.add(v);

        while(que.size() > 0){
            v = que.poll();
            if(!graph.containsKey(v)) continue;
            for(int next : graph.get(v)){
                if(dist[next] != -1) continue;
                dist[next] = dist[v] + 1;
                que.add(next);
            }
        }
    }
    // ダイクストラ法
    Node[] dijkstra(HashMap<Integer, List<Integer>> graph, int size, int v, boolean[] flag, int[][] cost){
        Queue<Node> queue = new PriorityQueue<>(new MyComparator());
        Node[] nodes = new Node[size];
        for(int i = 1; i < size; i++){
            nodes[i] = new Node(i, INF, -1);            // id, dist, pastを指定して初期化
        }
        /* for(int key : graph.keySet()){
            nodes[key] = new Node(key, INF, -1);    // id, dist, pastを指定して初期化
        } */
       
        nodes[v].dist = 0;  // 始点のコストを0に
        try{
            queue.add(nodes[v].clone());
        } catch(CloneNotSupportedException e){
            e.printStackTrace();
        }

        while(queue.size() > 0){
            Node now = queue.poll();        // 未確定の中で最もコストの低いノードを取り出す
            flag[now.id] = true;                // 確定
            if(!graph.containsKey(now.id)) continue;
            for(int next : graph.get(now.id)){      // 辺で繋がっているノードを順にチェック
                if(flag[next] || nodes[next].dist < nodes[now.id].dist + cost[now.id][next]) continue;
                nodes[next].dist = nodes[now.id].dist + cost[now.id][next];     // distを更新
                nodes[next].past = now.id;                                                    // pastを現在のノードに
                try{
                    queue.add(nodes[next].clone());
                } catch (CloneNotSupportedException e){
                    e.printStackTrace();
                }
            }
        }

        return nodes;
    }
    
    // ソート済みint型配列でkey以上の値の最小indexを返す
    int lowerBound(int[] a, int key){
        int ng = -1;
        int ok = a.length;
        while(Math.abs(ok - ng) > 1){
            int mid = (ok + ng) / 2; 
            if(a[mid] >= key){
                ok = mid;
            } else {
                ng = mid;
            }
        }
        return ok;
    }
    // ソート済みlong型配列でkey以上の値の最小indexを返す
    int lowerBound(long[] a, long key){
        int ng = -1;
        int ok = a.length;
        while(Math.abs(ok - ng) > 1){
            int mid = (ok + ng) / 2; 
            if(a[mid] >= key){
                ok = mid;
            } else {
                ng = mid;
            }
        }
        return ok;
    }
    // 文字列sとtの最長共通部分列の長さを返す
    int lcs(String s , String t){
        int[][] dp = new int[s.length() + 1][t.length() + 1];

        for(int i = 0; i < s.length(); i++){
            for(int j = 0; j < t.length(); j++){
                if(s.charAt(i) == t.charAt(j)){
                    dp[i + 1][j + 1] = max(dp[i][j] + 1, dp[i + 1][j + 1]);
                }
                dp[i + 1][j + 1] = max(dp[i + 1][j + 1], dp[i + 1][j]);
                dp[i + 1][j + 1] = max(dp[i + 1][j + 1], dp[i][j + 1]);
            }
        }

        return dp[s.length()][t.length()];
    }

}
// ダイクストラ法におけるノード
class Node implements Cloneable{
    int id;             // ノード番号
    int dist;           // 最低必要コスト
    int past;           // 直前の頂点

    Node(int id, int dist, int past){
        this.id = id;
        this.dist = dist;
        this.past = past;
    }
    @Override
    protected Node clone() throws CloneNotSupportedException{
        Node node = (Node)super.clone();
        return node;
    }
    public String toString(){
        return "[" + id + ", " + dist + " ," + past + "]";
    }
}
// ダイクストラ法における重み比較のComparator
class MyComparator implements Comparator<Node>{
    @Override
    public int compare(Node n1, Node n2){
        int dist1 = n1.dist;
        int dist2 = n2.dist;
        if(dist1 < dist2){
            return -1;
        } else if(dist1 > dist2){
            return 1;
        } else {
            return 0;
        }
    }
}
// 辺の情報を持つクラス
class Edge implements Cloneable{
    int from;       // どの頂点から
    int to;          //  どの頂点へ
    double cost;       // 辺の重み

    Edge(int from, int to, double cost){
        this.from = from;
        this.to = to;
        this.cost = cost;
    }
    @Override
    protected Edge clone() throws CloneNotSupportedException{
        Edge edge = (Edge)super.clone();
        return edge;
    }
    public String toString(){
        return "[" + from + " to " + to + "," + cost + "]";
    }
}
// 辺の重み比較のComparator
class EdgeComparator implements Comparator<Edge>{
    @Override
    public int compare(Edge e1, Edge e2){
        double cost1 = e1.cost;
        double cost2 = e2.cost;
        if(cost1 < cost2){
            return -1;
        } else if(cost1 > cost2){
            return 1;
        } else {
            return 0;
        }
    }
}
// Union-Find
class UnionFind{
    int[] par;
    int[] size;
    UnionFind(int N){
        par = new int[N];
        size = new int[N];
        for(int i = 0; i < N; i++){
            par[i] = i;
            size[i] = 1;
        }
    }

    void init(int N){
        for(int i = 0; i < N; i++){
            par[i] = i;
            size[i] = 1;
        }
    }

    int root(int x){
        if(par[x] == x){
            return x;
        } else {
            return par[x] = root(par[x]);
        }
    }

    boolean same(int x, int y){
        return root(x) == root(y);
    }

    void unite(int x, int y){
        x = root(x);
        y = root(y);
        if(x == y) return;
        if(size[x] < size[y]){
            int tmp = x;
            x = y;
            y = tmp;
        }
        size[x] += size[y];
        par[y] = x;
    }

    int size(int x){
        return size[root(x)];
    }

}

// 順列を管理する
class Permutation {
    private int number;
    private int listSize;
    private int searched;
    private int nextIndex;
    private int[][] permList;

    Permutation(int num) {
        this.number = num;
        this.listSize  = this.fact(this.number);
        this.searched   = 0;
        this.nextIndex = 0;
        this.permList  = new int[this.listSize][this.number];

        this.create(0, new int[this.number], new boolean[this.number]);
    }

    int[] nextPerm() {
        return permList[this.nextIndex++];
    }

    boolean isNext() {
        if(this.nextIndex < this.listSize) {
            return true;
        } else {
            this.nextIndex = 0;
            return false;
        }
    }

    int fact(int n){
        return n == 0 ? 1 : n * fact(n-1);
    }

    void create(int num, int[] list, boolean[] flag) {
        if(num == this.number) {
            copyArray(list, permList[this.searched]);
            this.searched++;
        }
        for(int i = 0; i < this.number; i++){
            if(flag[i]) continue;
            list[num] = i;
            flag[i] = true;
            this.create(num+1, list, flag);
            flag[i] = false;

        }
    }

    void copyArray(int[] from, int[] to) {
        for(int i=0; i<from.length; i++) to[i] = from[i];
    }

    void printNum(int[] nums) {
        for(int n : nums) System.out.print(n);
        System.out.println();
    }
}

// 標準のScannerより高速に標準入力する
class FastScanner {
    private final InputStream in = System.in;
    private final byte[] buffer = new byte[1024];
    private int ptr = 0;
    private int buflen = 0;
    private boolean hasNextByte() {
        if (ptr < buflen) {
            return true;
        }else{
            ptr = 0;
            try {
                buflen = in.read(buffer);
            } catch (IOException e) {
                e.printStackTrace();
            }
            if (buflen <= 0) {
                return false;
            }
        }
        return true;
    }
    private int readByte() { if (hasNextByte()) return buffer[ptr++]; else return -1;}
    private static boolean isPrintableChar(int c) { return 33 <= c && c <= 126;}
    public boolean hasNext() { while(hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++; return hasNextByte();}
    public String next() {
        if (!hasNext()) throw new NoSuchElementException();
        StringBuilder sb = new StringBuilder();
        int b = readByte();
        while(isPrintableChar(b)) {
            sb.appendCodePoint(b);
            b = readByte();
        }
        return sb.toString();
    }
    public long nextLong() {
        if (!hasNext()) throw new NoSuchElementException();
        long n = 0;
        boolean minus = false;
        int b = readByte();
        if (b == '-') {
            minus = true;
            b = readByte();
        }
        if (b < '0' || '9' < b) {
            throw new NumberFormatException();
        }
        while(true){
            if ('0' <= b && b <= '9') {
                n *= 10;
                n += b - '0';
            }else if(b == -1 || !isPrintableChar(b)){
                return minus ? -n : n;
            }else{
                throw new NumberFormatException();
            }
            b = readByte();
        }
    }
    public int nextInt() {
        long nl = nextLong();
        if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE) throw new NumberFormatException();
        return (int) nl;
    }
    public double nextDouble() { return Double.parseDouble(next());}
}