Merge pull request #2 from SankarshanMishra/SankarshanMishra-patch-2

Create Greedy Algorithm For Graph Coloring

Author: SankarshanMishra

greedy algorithm for graph coloring

@@ -0,0 +1,105 @@
+// A Java program to implement greedy algorithm for graph coloring
+import java.io.*;
+import java.util.*;
+import java.util.LinkedList;
+
+// This class represents an undirected graph using adjacency list
+class Graph
+{
+	private int V; // No. of vertices
+	private LinkedList<Integer> adj[]; //Adjacency List
+
+	//Constructor
+	Graph(int v)
+	{
+		V = v;
+		adj = new LinkedList[v];
+		for (int i=0; i<v; ++i)
+			adj[i] = new LinkedList();
+	}
+
+	//Function to add an edge into the graph
+	void addEdge(int v,int w)
+	{
+		adj[v].add(w);
+		adj[w].add(v); //Graph is undirected
+	}
+
+	// Assigns colors (starting from 0) to all vertices and
+	// prints the assignment of colors
+	void greedyColoring()
+	{
+		int result[] = new int[V];
+
+		// Initialize all vertices as unassigned
+		Arrays.fill(result, -1);
+
+		// Assign the first color to first vertex
+		result[0] = 0;
+
+		// A temporary array to store the available colors. False
+		// value of available[cr] would mean that the color cr is
+		// assigned to one of its adjacent vertices
+		boolean available[] = new boolean[V];
+		
+		// Initially, all colors are available
+		Arrays.fill(available, true);
+
+		// Assign colors to remaining V-1 vertices
+		for (int u = 1; u < V; u++)
+		{
+			// Process all adjacent vertices and flag their colors
+			// as unavailable
+			Iterator<Integer> it = adj[u].iterator() ;
+			while (it.hasNext())
+			{
+				int i = it.next();
+				if (result[i] != -1)
+					available[result[i]] = false;
+			}
+
+			// Find the first available color
+			int cr;
+			for (cr = 0; cr < V; cr++){
+				if (available[cr])
+					break;
+			}
+
+			result[u] = cr; // Assign the found color
+
+			// Reset the values back to true for the next iteration
+			Arrays.fill(available, true);
+		}
+
+		// print the result
+		for (int u = 0; u < V; u++)
+			System.out.println("Vertex " + u + " ---> Color "
+								+ result[u]);
+	}
+
+	// Driver method
+	public static void main(String args[])
+	{
+		Graph g1 = new Graph(5);
+		g1.addEdge(0, 1);
+		g1.addEdge(0, 2);
+		g1.addEdge(1, 2);
+		g1.addEdge(1, 3);
+		g1.addEdge(2, 3);
+		g1.addEdge(3, 4);
+		System.out.println("Coloring of graph 1");
+		g1.greedyColoring();
+
+		System.out.println();
+		Graph g2 = new Graph(5);
+		g2.addEdge(0, 1);
+		g2.addEdge(0, 2);
+		g2.addEdge(1, 2);
+		g2.addEdge(1, 4);
+		g2.addEdge(2, 4);
+		g2.addEdge(4, 3);
+		System.out.println("Coloring of graph 2 ");
+		g2.greedyColoring();
+	}
+}
+// This code is contributed by Aakash Hasija