Vizing’s Theorem

In graph theory, Vizing’s theorem states that every simple undirected graph may be edge colored using a number of colors that is at most one larger than the maximum degree ‘d’ of the graph. In simple meaning this theorem states that the chromatic index of the simple graph can be either ‘d’ or ‘d’ +1. The minimum number of colors needed for the edge coloring of the graph is called chromatic index. 

There are 5 vertices in the above graph G. Highest Degree is 4, but we need 5 colors, so that no edge shares the same color with any edge of the adjacent vertices, as you can see in the above graph. Therefore the required number of valid colors for this graph is equal to 5, which is ( ‘highest degree’ + 1 ). 
Note: c1, c2, c3, c4 and c5 in the above diagram implies distinct colors.

Examples :

Input : 
v = 3, e = 3 
{{ 1, 2, -1 }, 
{ 2, 3, -1 }, 
{ 3, 1, -1 }}; 
Output : 
3 colors needs to generate a valid edge coloring :
color between v(1): 1 and v(2): 2 is: color 1 
color between v(1): 2 and v(2): 3 is: color 2 
color between v(1): 3 and v(2): 1 is: color 3

Algorithm: 

• After initializing the number of edges, assign the vertex pair of every edge
• Color the graph edges according to the theorem
• Assign a color and then check its validity
• Check if any two adjacent edges have the same color, then increment the Color, goto flag and try next color
• Repeat till all the edges get it’s color according to the theorem
• Once done print the color of all the edges between the vertices

Below is the implementation of the above approach: 

C++

// C++ program to illustrate
// Vizing's Theorem
#include <iostream>
using namespace std;

// Function to print the color of the edge
void colorEdge(int edges[][3], int e)
{
    int color;

    // Assign a color to every edge 'i'.
    for (int i = 0; i < e; i++) {
        color = 1;
    flag:
        // Assign a color and
        // then check its validity.
        edges[i][2] = color;
        for (int j = 0; j < e; j++) {
            if (j == i)
                continue;

            // If the color of edges
            // is adjacent to edge i
            if ((edges[i][0] == edges[j][0])
                || (edges[i][1] == edges[j][0])
                || (edges[i][0] == edges[j][1])
                || (edges[i][1] == edges[j][1])) {

                // If the color matches
                if (edges[i][2] == edges[j][2]) {

                    // Increment the color,
                    // denotes the change in color.
                    color++;

                    // goes back, and assigns
                    // the next color.
                    goto flag;
                }
            }
        }
    }
    // Check the maximum color from all the edge colors
    int maxColor = -1;
    for (int i = 0; i < e; i++) {
        maxColor = max(maxColor, edges[i][2]);
    }
    cout << maxColor
         << " colors needs to generate a valid edge "
            "coloring:"
         << endl;
    for (int i = 0; i < e; i++) {
        cout << "color between v(1): " << edges[i][0]
             << " and v(2): " << edges[i][1]
             << " is: color " << edges[i][2] << endl;
    }
}

// Driver Code
int main()
{
    // initialize the number
    // of edges of the graph
    int e = 5;

    // initialize the vertex
    // pair of every edge in a graph
    int edges[e][3] = { { 1, 2, -1 },
                        { 2, 3, -1 },
                        { 3, 4, -1 },
                        { 4, 1, -1 },
                        { 1, 3, -1 } };

    colorEdge(edges, e);
    return 0;
}

Java

// Java program to illustrate
// Vizing's Theorem
import java.util.*;

public class VizingsTheorem {

    // Function to find the chromatic index
    public void colorEdge(int[][] edges, int e)
    {
        // Initialize edge to first edge and
        // color to color 1
        int i = 0, color = 1;

        // Repeat until all edges are done coloring
        while (i < e) {
            // Give the selected edge a color
            edges[i][2] = color;
            boolean flag = false;
            // Iterate through all others edges to check
            for (int j = 0; j < e; j++) {
                // Ignore if same edge
                if (j == i)
                    continue;
                // Check if one vertex is similar
                if ((edges[i][0] == edges[j][0])
                    || (edges[i][1] == edges[j][0])
                    || (edges[i][0] == edges[j][1])
                    || (edges[i][1] == edges[j][1])) {
                    // Check if color is similar
                    if (edges[i][2] == edges[j][2]) {
                        // Increment the color by 1
                        color++;
                        flag = true;
                        break;
                    }
                }
            }

            // If same color faced then repeat again
            if (flag == true) {
                continue;
            }

            // Or else proceed to a
            // new vertex with color 1
            color = 1;
            i++;
        }

        // Check the maximum color from all the edge colors
        int maxColor = -1;
        for (i = 0; i < e; i++)
        {
            maxColor = Math.max(maxColor, edges[i][2]);
        }
        System.out.println(
            maxColor
            + " colors needs to generate"
            +" a valid edge coloring:");
        for (i = 0; i < e; i++)
        {
            System.out.println(
                "color between v(1): " + edges[i][0]
                + " and v(2): " + edges[i][1]
                + " is: color " + edges[i][2]);
        }
    }

    // Driver code
    public static void main(String[] args)
    {

        // Number of edges
        int e = 5;

        // Edge list
        int[][] edges = new int[e][3];

        // Initialize all edge colors to 0
        for (int i = 0; i < e; i++) {
            edges[i][2] = -1;
        }

        // Edges
        edges[0][0] = 1;
        edges[0][1] = 2;

        edges[1][0] = 2;
        edges[1][1] = 3;

        edges[2][0] = 3;
        edges[2][1] = 4;

        edges[3][0] = 4;
        edges[3][1] = 1;

        edges[4][0] = 1;
        edges[4][1] = 3;

        // Run the function
        VizingsTheorem c = new VizingsTheorem();
        c.colorEdge(edges, e);
    }
}

Python3

def colorEdge(edges, e):
    # Initialize edge to first edge and
    # color to color 1
    i = 0
    color = 1
    # Repeat until all edges are done coloring
    while(i < e):
        # Give the selected edge a color
        edges[i][2] = color
        flag = False
        # Iterate through all others edges to check
        for j in range(e):
            # Ignore if same edge
            if (j == i):
                continue
            # Check if one vertex is similar
            if ((edges[i][0] == edges[j][0])or
                (edges[i][1] == edges[j][0]) or
                (edges[i][0] == edges[j][1]) or
                    (edges[i][1] == edges[j][1])):
                # Check if color is similar
                if (edges[i][2] == edges[j][2]):
                    # Increment the color by 1
                    color += 1
                    flag = True
                    break
        # If same color faced then repeat again
        if (flag == True):
            continue

        # Or else proceed to a new vertex with color 1
        color = 1
        i += 1

    # Check the maximum color from all the edge colors
    maxColor = -1
    for i in range(e):
        maxColor = max(maxColor, edges[i][2])
    print(str(maxColor)+" colors needs to generate a valid edge coloring:")
    for i in range(e):
        print("color between v(1): "+str(edges[i][0])+" and v(2): "
              + str(edges[i][1])+" is: color "+str(edges[i][2]))


# Driver code

if __name__ == "__main__":
    # Number of edges
    e = 5
    # Edge list
    edges = [[0 for _ in range(3)] for _ in range(e)]
    # Initialize all edge colors to 0
    for i in range(e):
        edges[i][2] = -1
    # Edges
    edges[0][0] = 1
    edges[0][1] = 2

    edges[1][0] = 2
    edges[1][1] = 3

    edges[2][0] = 3
    edges[2][1] = 4

    edges[3][0] = 4
    edges[3][1] = 1

    edges[4][0] = 1
    edges[4][1] = 3

    # Run the function
    colorEdge(edges, e)

C#

using System;

public class VizingsTheorem
{
    // Function to find the chromatic index
    public void colorEdge(int[,] edges, int e)
    {
        // Initialize edge to first edge and color to color 1
        int i = 0, color = 1;

        // Repeat until all edges are done coloring
        while (i < e)
        {
            // Give the selected edge a color
            edges[i, 2] = color;
            bool flag = false;

            // Iterate through all others edges to check
            for (int j = 0; j < e; j++)
            {
                // Ignore if same edge
                if (j == i)
                    continue;

                // Check if one vertex is similar
                if ((edges[i, 0] == edges[j, 0])
                    || (edges[i, 1] == edges[j, 0])
                    || (edges[i, 0] == edges[j, 1])
                    || (edges[i, 1] == edges[j, 1]))
                {
                    // Check if color is similar
                    if (edges[i, 2] == edges[j, 2])
                    {
                        // Increment the color by 1
                        color++;
                        flag = true;
                        break;
                    }
                }
            }

            // If same color faced then repeat again
            if (flag == true)
                continue;

            // Or else proceed to a new vertex with color 1
            color = 1;
            i++;
        }

        // Check the maximum color from all the edge colors
        int maxColor = -1;
        for (i = 0; i < e; i++)
        {
            maxColor = Math.Max(maxColor, edges[i, 2]);
        }
        Console.WriteLine(maxColor + " colors needs to generate a valid edge coloring:");
        for (i = 0; i < e; i++)
        {
            Console.WriteLine("color between v(1): " + edges[i, 0] + " and v(2): " + edges[i, 1] + " is: color " + edges[i, 2]);
        }
    }

    // Driver code
    public static void Main(string[] args)
    {
        // Number of edges
        int e = 5;

        // Edge list
        int[,] edges = new int[e, 3];

        // Initialize all edge colors to 0
        for (int i = 0; i < e; i++)
            edges[i, 2] = -1;

        // Edges
        edges[0, 0] = 1;
        edges[0, 1] = 2;

        edges[1, 0] = 2;
        edges[1, 1] = 3;

        edges[2, 0] = 3;
        edges[2, 1] = 4;

        edges[3, 0] = 4;
        edges[3, 1] = 1;

        edges[4, 0] = 1;
        edges[4, 1] = 3;

        // Run the function
        VizingsTheorem c = new VizingsTheorem();
        c.colorEdge(edges, e);
    }
}

Javascript

<script>

// JavaScript program to illustrate
// Vizing's Theorem
// Function to find the chromatic index
function colorEdge(edges, e)
{
    // Initialize edge to first edge and
    // color to color 1
    var i = 0, color = 1;
    // Repeat until all edges are done coloring
    while (i < e) {
        // Give the selected edge a color
        edges[i][2] = color;
        var flag = false;
        // Iterate through all others edges to check
        for (var j = 0; j < e; j++) {
            // Ignore if same edge
            if (j == i)
                continue;
            // Check if one vertex is similar
            if ((edges[i][0] == edges[j][0])
                || (edges[i][1] == edges[j][0])
                || (edges[i][0] == edges[j][1])
                || (edges[i][1] == edges[j][1])) {
                // Check if color is similar
                if (edges[i][2] == edges[j][2]) {
                    // Increment the color by 1
                    color++;
                    flag = true;
                    break;
                }
            }
        }
        // If same color faced then repeat again
        if (flag == true) {
            continue;
        }
        // Or else proceed to a
        // new vertex with color 1
        color = 1;
        i++;
    }
    // Check the maximum color from all the edge colors
    var maxColor = -1;
    for (i = 0; i < e; i++)
    {
        maxColor = Math.max(maxColor, edges[i][2]);
    }
    document.write(
        maxColor
        + " colors needs to generate"
        +" a valid edge coloring:<br>");
    for (i = 0; i < e; i++)
    {
        document.write(
            "color between v(1): " + edges[i][0]
            + " and v(2): " + edges[i][1]
            + " is: color " + edges[i][2] + "<br>");
    }
}
// Driver code
// Number of edges
var e = 5;
// Edge list
var edges = Array.from(Array(e), ()=>Array(3));
// Initialize all edge colors to 0
for (var i = 0; i < e; i++) {
    edges[i][2] = -1;
}
// Edges
edges[0][0] = 1;
edges[0][1] = 2;
edges[1][0] = 2;
edges[1][1] = 3;
edges[2][0] = 3;
edges[2][1] = 4;
edges[3][0] = 4;
edges[3][1] = 1;
edges[4][0] = 1;
edges[4][1] = 3;

// Run the function
colorEdge(edges, e);


</script>

Output

3 colors needs to generate a valid edge coloring:
color between v(1): 1 and v(2): 2 is: color 1
color between v(1): 2 and v(2): 3 is: color 2
color between v(1): 3 and v(2): 4 is: color 1
color between v(1): 4 and v(2): 1 is: color 2
color between v(1): 1 and v(2): 3 is: color 3

Time Complexity: O(e²)

Auxiliary Space: O(1)

As constant extra space is used.