programming-examples/java/Searching_Sorting_Algorithms/Java Program to Create the Prufer Code for a Tree.java

/*This is a java program to create Prufer code for a tree. In combinatorial mathematics, the Prüfer sequence (also Prüfer code or Prüfer numbers) of a labeled tree is a unique sequence associated with the tree. The sequence for a tree on n vertices has length n – 2, and can be generated by a simple iterative algorithm.*/


import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Random;
import java.util.Scanner;
import java.util.Set;

public class PruferCode
{
    public static List<Integer>[] getRandomTree2(int n, Random rnd)
    {
        @SuppressWarnings("unchecked")
        List<Integer>[] t = new List[n];
        for (int i = 0; i < n; i++)
            t[i] = new ArrayList<>();
        int[] p = new int[n];
        for (int i = 0, j; i < n; j = rnd.nextInt(i + 1), p[i] = p[j], p[j] = i, i++)
            ; // random permutation
        for (int i = 1; i < n; i++)
            {
                int parent = p[rnd.nextInt(i)];
                t[parent].add(p[i]);
                t[p[i]].add(parent);
            }
        return t;
    }

    public static List<Integer>[] pruferCode2Tree(int[] pruferCode)
    {
        int n = pruferCode.length + 2;
        @SuppressWarnings("unchecked")
        List<Integer>[] tree = new List[n];
        for (int i = 0; i < n; i++)
            tree[i] = new ArrayList<>();
        int[] degree = new int[n];
        Arrays.fill(degree, 1);
        for (int v : pruferCode)
            ++degree[v];
        int ptr = 0;
        while (degree[ptr] != 1)
            ++ptr;
        int leaf = ptr;
        for (int v : pruferCode)
            {
                tree[leaf].add(v);
                tree[v].add(leaf);
                --degree[leaf];
                --degree[v];
                if (degree[v] == 1 && v < ptr)
                    {
                        leaf = v;
                    }
                else
                    {
                        for (++ptr; ptr < n && degree[ptr] != 1; ++ptr)
                            ;
                        leaf = ptr;
                    }
            }
        for (int v = 0; v < n - 1; v++)
            {
                if (degree[v] == 1)
                    {
                        tree[v].add(n - 1);
                        tree[n - 1].add(v);
                    }
            }
        return tree;
    }

    public static int[] tree2PruferCode(List<Integer>[] tree)
    {
        int n = tree.length;
        int[] parent = new int[n];
        parent[n - 1] = -1;
        pruferDfs(tree, parent, n - 1);
        int[] degree = new int[n];
        int ptr = -1;
        for (int i = 0; i < n; ++i)
            {
                degree[i] = tree[i].size();
                if (degree[i] == 1 && ptr == -1)
                    ptr = i;
            }
        int[] res = new int[n - 2];
        int leaf = ptr;
        for (int i = 0; i < n - 2; ++i)
            {
                int next = parent[leaf];
                res[i] = next;
                --degree[next];
                if (degree[next] == 1 && next < ptr)
                    {
                        leaf = next;
                    }
                else
                    {
                        ++ptr;
                        while (ptr < n && degree[ptr] != 1)
                            ++ptr;
                        leaf = ptr;
                    }
            }
        return res;
    }

    static void pruferDfs(List<Integer>[] tree, int[] parent, int v)
    {
        for (int i = 0; i < tree[v].size(); ++i)
            {
                int to = tree[v].get(i);
                if (to != parent[v])
                    {
                        parent[to] = v;
                        pruferDfs(tree, parent, to);
                    }
            }
    }

    // precondition: n >= 2
    public static List<Integer>[] getRandomTree(int V, Random rnd)
    {
        int[] a = new int[V - 2];
        for (int i = 0; i < a.length; i++)
            {
                a[i] = rnd.nextInt(V);
            }
        return pruferCode2Tree(a);
    }

    // precondition: V >= 2, V-1 <= E <= V*(V-1)/2
    public static List<Integer>[] getRandomUndirectedConnectedGraph(int V,
            int E, Random rnd)
    {
        List<Integer>[] g = getRandomTree(V, rnd);
        Set<Long> edgeSet = new LinkedHashSet<>();
        for (int i = 0; i < V; i++)
            {
                for (int j = i + 1; j < V; j++)
                    {
                        edgeSet.add(((long) i << 32) + j);
                    }
            }
        for (int i = 0; i < V; i++)
            {
                for (int j : g[i])
                    {
                        edgeSet.remove(((long) i << 32) + j);
                    }
            }
        List<Long> edges = new ArrayList<>(edgeSet);
        for (int x : getRandomArrangement(edges.size(), E - (V - 1), rnd))
            {
                long e = edges.get(x);
                int u = (int) (e >>> 32);
                int v = (int) e;
                g[u].add(v);
                g[v].add(u);
            }
        for (int i = 0; i < V; i++)
            Collections.sort(g[i]);
        return g;
    }

    // precondition: V >= 2, V-1 <= E <= V*(V-1)/2
    public static List<Integer>[] getRandomUndirectedConnectedGraph2(int V,
            int E, Random rnd)
    {
        List<Integer>[] g = getRandomTree(V, rnd);
        Set<Long> edgeSet = new LinkedHashSet<>();
        for (int i = 0; i < V; i++)
            {
                for (int j : g[i])
                    {
                        edgeSet.add(((long) i << 32) + j);
                    }
            }
        for (int i = 0; i < E - (V - 1); i++)
            {
                int u;
                int v;
                long edge;
                while (true)
                    {
                        u = rnd.nextInt(V);
                        v = rnd.nextInt(V);
                        edge = ((long) u << 32) + v;
                        if (u < v && !edgeSet.contains(edge))
                            break;
                    }
                edgeSet.add(edge);
                g[u].add(v);
                g[v].add(u);
            }
        for (int i = 0; i < V; i++)
            Collections.sort(g[i]);
        return g;
    }

    static int[] getRandomArrangement(int n, int m, Random rnd)
    {
        int[] res = new int[n];
        for (int i = 0; i < n; i++)
            {
                res[i] = i;
            }
        for (int i = 0; i < m; i++)
            {
                int j = n - 1 - rnd.nextInt(n - i);
                int t = res[i];
                res[i] = res[j];
                res[j] = t;
            }
        return Arrays.copyOf(res, m);
    }

    static void checkGraph(int V, int E, Random rnd)
    {
        List<Integer>[] g = getRandomUndirectedConnectedGraph(V, E, rnd);
        int n = g.length;
        int[][] a = new int[n][n];
        int edges = 0;
        for (int i = 0; i < n; i++)
            {
                for (int j : g[i])
                    {
                        ++a[i][j];
                        ++edges;
                    }
            }
        if (edges != 2 * E)
            {
                throw new RuntimeException();
            }
        for (int i = 0; i < n; i++)
            {
                if (a[i][i] != 0)
                    {
                        throw new RuntimeException();
                    }
                for (int j = 0; j < n; j++)
                    {
                        if (a[i][j] != a[j][i] || a[i][j] != 0 && a[i][j] != 1)
                            {
                                throw new RuntimeException();
                            }
                    }
            }
    }

    public static void main(String[] args)
    {
        Scanner sc = new Scanner(System.in);
        System.out.println("Enter the length of code: ");
        int n = sc.nextInt();
        int[] code = new int[n];
        for (int i = 0; i < n; i++)
            {
                code[i] = sc.nextInt();
            }
        List<Integer>[] tree = pruferCode2Tree(code);
        Random rnd = new Random(1);
        for (int step = 0; step < 1000; step++)
            {
                int V = rnd.nextInt(50) + 2;
                checkGraph(V, V - 1, rnd);
                checkGraph(V, V * (V - 1) / 2, rnd);
                checkGraph(V, rnd.nextInt(V * (V - 1) / 2 - (V - 1) + 1) + V - 1,
                           rnd);
            }
        System.out.println("Prufer code to tree conversion: "
                           + Arrays.toString(tree));
        System.out.println("Tree to prufer code conversion: "
                           + Arrays.toString(tree2PruferCode(tree)));
        sc.close();
    }
}

/*
Enter the length of code:
4
2 3 4 3
Prufer code to tree conversion: [[2], [3], [0, 4], [1, 4, 5], [2, 3], [3]]
Tree to prufer code conversion: [2, 3, 4, 3]

Enter the length of code:
3
2 4 3
Prufer code to tree conversion: [[2], [4], [0, 3], [2, 4], [1, 3]]
Tree to prufer code conversion: [2, 4, 3]

Enter the length of code:
5
3 4 5 1 6
Prufer code to tree conversion: [[3], [4, 6], [4], [0, 5], [2, 1], [3, 6], [1, 5]]
Tree to prufer code conversion: [3, 4, 5, 1, 6]