# Graph Coloring

Data StructureGraph AlgorithmsAlgorithms

Best Seller

89 Lectures 11.5 hours

#### Practical Data Science using Python

22 Lectures 6 hours

#### Data Science and Data Analysis with Python

50 Lectures 3.5 hours

Graph coloring problem is a special case of graph labeling. In this problem, each node is colored into some colors. But coloring has some constraints. We cannot use the same color for any adjacent vertices.

For solving this problem, we need to use the greedy algorithm, but it does not guaranty to use minimum color.

## Input and Output

Input:
0 0 1 0 1
0 0 1 1 1
1 1 0 1 0
0 1 1 0 1
1 1 0 1 0

Output:
Node: 0, Assigned with Color: 0
Node: 1, Assigned with Color: 0
Node: 2, Assigned with Color: 1
Node: 3, Assigned with Color: 2
Node: 4, Assigned with Color: 1

## Algorithm

graphColoring(graph)

Input − The given graph.

Output − Each node with some color assigned to it.

Begin
declare a list of colors
initially set the color 0 for first node
define an array colorUsed to track which color is used, and which colors have never used.

for all vertices i except first one, do
mark i as unassigned to any color
done

mark colorUsed to false for all vertices
for all vertices u in the graph except 1st vertex, do
for all vertex v adjacent with u, do
if color[v] is unassigned, then
mark colorUsed[color[v]] := true
done

for all colors col in the color list, do
if color is not used, then
stop the loop
done

color[u] := col
for each vertex v which is adjacent with u, do
if color[v] is unassigned, then
colorUsed[color[v]] := false
done
done

for all vertices u in the graph, do
display the node and its color
done
End

## Example

#include<iostream>
#define NODE 6
using namespace std;

int graph[NODE][NODE] = {
{0, 1, 1, 1, 0, 0},
{1, 0, 0, 1, 1, 0},
{1, 0, 0, 1, 0, 1},
{1, 1, 1, 0, 1, 1},
{0, 1, 0, 1, 0, 1},
{0, 0, 1, 1, 1, 0}
};

void graphColoring() {
int color[NODE];
color[0] = 0;    //Assign first color for the first node
bool colorUsed[NODE];    //Used to check whether color is used or not

for(int i = 1; i<NODE; i++)
color[i] = -1;    //initialize all other vertices are unassigned

for(int i = 0; i<NODE; i++)
colorUsed[i] = false;    //initially any colors are not chosen

for(int u = 1; u<NODE; u++) {    //for all other NODE - 1 vertices
for(int v = 0; v<NODE; v++) {
if(graph[u][v]){
if(color[v] != -1)    //when one color is assigned, make it unavailable
colorUsed[color[v]] = true;
}
}

int col;
for(col = 0; col<NODE; col++)
if(!colorUsed[col])    //find a color which is not assigned
break;

color[u] = col;    //assign found color in the list

for(int v = 0; v<NODE; v++) {    //for next iteration make color availability to false
if(graph[u][v]) {
if(color[v] != -1)
colorUsed[color[v]] = false;
}
}
}

for(int u = 0; u<NODE; u++)
cout <<"Color: " << u << ", Assigned with Color: " <<color[u] <<endl;
}

main() {
graphColoring();
}

## Output

Node: 0, Assigned with Color: 0
Node: 1, Assigned with Color: 0
Node: 2, Assigned with Color: 1
Node: 3, Assigned with Color: 2
Node: 4, Assigned with Color: 1
Updated on 16-Jun-2020 12:53:49