Prufer Code to Tree Creation

• Difficulty Level : Medium
• Last Updated : 13 Jan, 2022

What is Prufer Code?
Given a tree (represented as graph, not as a rooted tree) with n labeled nodes with labels from 1 to n, a Prufer code uniquely identifies the tree. The sequence has n-2 values.
How to get Prufer Code of a tree?

1. Initialize Prufer code as empty.
2. Start with a leaf of lowest label say x. Find the vertex connecting it to the rest of tree say y. Remove x from the tree and add y to the Prufer Code
3. Repeat above step 2 until we are left with two nodes.

A tree with labels from 1 to n.
5
/   \
1     4
/  \
2    3

PruferCode = {}
The lowest label leaf is 2, we remove it from tree
and add the other vertex (connecting it to the tree)
to Prufer code
Tree now becomes
5
/   \
1     4
\
3
Prufer Code becomes = {1}

The lowest label leaf is 3, we remove it from tree
and add the other vertex (connecting it to the tree)
to Prufer code
Tree now becomes
5
/   \
1     4
Prufer Code becomes = {1, 1}

The lowest label leaf is 1, we remove it from tree
and add the other vertex (connecting it to the tree)
to Prufer code
Tree now becomes
5
\
4
Prufer Code becomes = {1, 1, 5}

We have only two nodes left now, so we stop.

How to construct a tree from given Prufer Code?

Input : (4, 1, 3, 4)
Output : Edges of following tree
2----4----3----1----5
|
6

Input : (1, 3, 5)
Output : Edges of following tree
2----1----3----5----4

Let the length of given Prufer code be m. The idea is to create an empty graph of m+2 vertices. We remove first element from sequence. Let first element of current sequence be x. Then we find the least value which is not present in the given sequence and not yet added to the tree. Let this value be be y. We add an edge from x to y and repeat this step.
Let us understand algorithm to construct tree with above first example:

Input : (4, 1, 3, 4)

Step 1: First we create an empty graph of 6 vertices
and get 4 from the sequence.
Step 2: Out of 1 to 6, the least vertex not in
Prufer sequence is 2.
Step 3: We form an edge between 2 and 4.
2----4    1    3    5      6
Step 4: Next in the sequence is 1 and corresponding
vertex with least degree is 5 (as 2 has been
considered).
2----4    1----5    3    6
Step 5: Next in the sequence is 3 and corresponding
vertex with least degree is 1
(as 1 is now not part of remaining Prufer sequence)
2----4    3----1----5    6
Step 6: Next in the sequence is 4 and corresponding vertex
with least degree is 3 (as 3 has not been considered
as is not present further in sequence)
2----4----3----1----5    6
Step 7: Finally two vertices are left out from 1 to 6 (4
and 6) so we join them.
2----4----3----1----5
|
6
This is the required tree on 6 vertices.

Following is the implementation.

C++

 // C++ program to construct tree from given Prufer Code#include using namespace std;  // Prints edges of tree represented by give Prufer codevoid printTreeEdges(int prufer[], int m){    int vertices = m + 2;    int vertex_set[vertices];      // Initialize the array of vertices    for (int i = 0; i < vertices; i++)        vertex_set[i] = 0;      // Number of occurrences of vertex in code    for (int i = 0; i < vertices - 2; i++)        vertex_set[prufer[i] - 1] += 1;      cout << "\nThe edge set E(G) is :\n";      // Find the smallest label not present in    // prufer[].    int j = 0;    for (int i = 0; i < vertices - 2; i++) {        for (j = 0; j < vertices; j++) {            // If j+1 is not present in prufer set            if (vertex_set[j] == 0) {                // Remove from Prufer set and print                // pair.                vertex_set[j] = -1;                cout << "(" << (j + 1) << ", "                     << prufer[i] << ")  ";                  vertex_set[prufer[i] - 1]--;                  break;            }        }    }      j = 0;    // For the last element    for (int i = 0; i < vertices; i++) {        if (vertex_set[i] == 0 && j == 0) {            cout << "(" << (i + 1) << ", ";            j++;        }        else if (vertex_set[i] == 0 && j == 1)            cout << (i + 1) << ")\n";    }}  // Driver codeint main(){    int prufer[] = { 4, 1, 3, 4 };    int n = sizeof(prufer) / sizeof(prufer);    printTreeEdges(prufer, n);    return 0;}

Java

 // Java program to construct tree from given Prufer Codeclass GFG {      // Prints edges of tree represented by give Prufer code    static void printTreeEdges(int prufer[], int m)    {        int vertices = m + 2;        int vertex_set[] = new int[vertices];          // Initialize the array of vertices        for (int i = 0; i < vertices; i++)            vertex_set[i] = 0;          // Number of occurrences of vertex in code        for (int i = 0; i < vertices - 2; i++)            vertex_set[prufer[i] - 1] += 1;          System.out.print("\nThe edge set E(G) is :\n");          // Find the smallest label not present in        // prufer[].        int j = 0;        for (int i = 0; i < vertices - 2; i++) {            for (j = 0; j < vertices; j++) {                // If j+1 is not present in prufer set                if (vertex_set[j] == 0) {                    // Remove from Prufer set and print                    // pair.                    vertex_set[j] = -1;                    System.out.print("(" + (j + 1) + ", "                                     + prufer[i] + ") ");                      vertex_set[prufer[i] - 1]--;                      break;                }            }        }          j = 0;        // For the last element        for (int i = 0; i < vertices; i++) {            if (vertex_set[i] == 0 && j == 0) {                System.out.print("(" + (i + 1) + ", ");                j++;            }            else if (vertex_set[i] == 0 && j == 1)                System.out.print((i + 1) + ")\n");        }    }      // Driver code    public static void main(String args[])    {        int prufer[] = { 4, 1, 3, 4 };        int n = prufer.length;        printTreeEdges(prufer, n);    }}  // This code is contributed by Arnab Kundu

Python3

 # Python3 program to construct # tree from given Prufer Code   # Prints edges of tree represented# by give Prufer code def printTreeEdges(prufer, m):          vertices = m + 2          # Initialize the array of vertices     vertex_set =  * vertices          # Number of occurrences of vertex in code     for i in range(vertices - 2):        vertex_set[prufer[i] - 1] += 1          print("The edge set E(G) is :")          # Find the smallest label not present in     # prufer.     j = 0    for i in range(vertices - 2):        for j in range(vertices):                          # If j+1 is not present in prufer set             if (vertex_set[j] == 0):                                  # Remove from Prufer set and print                 # pair.                 vertex_set[j] = -1                print("(" , (j + 1),", ",prufer[i],") ",sep = "",end = "")                vertex_set[prufer[i] - 1] -= 1                break          j = 0          # For the last element     for i in range(vertices):        if (vertex_set[i] == 0 and j == 0):            print("(", (i + 1),", ", sep="", end="")            j += 1        elif (vertex_set[i] == 0 and j == 1):            print((i + 1),")")  # Driver code prufer = [4, 1, 3, 4]n = len(prufer) printTreeEdges(prufer, n)   # This code is contributed by SHUBHAMSINGH10

C#

 // C# program to construct tree from given Prufer Codeusing System;  class GFG {      // Prints edges of tree represented by give Prufer code    static void printTreeEdges(int[] prufer, int m)    {        int vertices = m + 2;        int[] vertex_set = new int[vertices];          // Initialize the array of vertices        for (int i = 0; i < vertices; i++)            vertex_set[i] = 0;          // Number of occurrences of vertex in code        for (int i = 0; i < vertices - 2; i++)            vertex_set[prufer[i] - 1] += 1;          Console.Write("\nThe edge set E(G) is :\n");          // Find the smallest label not present in        // prufer[].        int j = 0;        for (int i = 0; i < vertices - 2; i++) {            for (j = 0; j < vertices; j++) {                // If j+1 is not present in prufer set                if (vertex_set[j] == 0) {                    // Remove from Prufer set and print                    // pair.                    vertex_set[j] = -1;                    Console.Write("(" + (j + 1) + ", "                                  + prufer[i] + ") ");                      vertex_set[prufer[i] - 1]--;                      break;                }            }        }          j = 0;        // For the last element        for (int i = 0; i < vertices; i++) {            if (vertex_set[i] == 0 && j == 0) {                Console.Write("(" + (i + 1) + ", ");                j++;            }            else if (vertex_set[i] == 0 && j == 1)                Console.Write((i + 1) + ")\n");        }    }      // Driver code    public static void Main(String[] args)    {        int[] prufer = { 4, 1, 3, 4 };        int n = prufer.Length;        printTreeEdges(prufer, n);    }}  // This code has been contributed by 29AjayKumar

Javascript



Output:

The edge set E(G) is :
(2, 4) (5, 1) (1, 3) (3, 4) (4, 6)

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