Dijkstra’ssolution.java

// Java Program to Implement Dijkstra's Algorithm
// Using Priority Queue

// Importing utility classes
import java.util.*;

// Main class DPQ
public class GFG {

	// Member variables of this class
	private int dist[];
	private Set<Integer> settled;
	private PriorityQueue<Node> pq;
	// Number of vertices
	private int V;
	List<List<Node> > adj;

	// Constructor of this class
	public GFG(int V)
	{

		// This keyword refers to current object itself
		this.V = V;
		dist = new int[V];
		settled = new HashSet<Integer>();
		pq = new PriorityQueue<Node>(V, new Node());
	}

	// Method 1
	// Dijkstra's Algorithm
	public void dijkstra(List<List<Node> > adj, int src)
	{
		this.adj = adj;

		for (int i = 0; i < V; i++)
			dist[i] = Integer.MAX_VALUE;

		// Add source node to the priority queue
		pq.add(new Node(src, 0));

		// Distance to the source is 0
		dist[src] = 0;

		while (settled.size() != V) {

			// Terminating condition check when
			// the priority queue is empty, return
			if (pq.isEmpty())
				return;

			// Removing the minimum distance node
			// from the priority queue
			int u = pq.remove().node;

			// Adding the node whose distance is
			// finalized
			if (settled.contains(u))

				// Continue keyword skips execution for
				// following check
				continue;

			// We don't have to call e_Neighbors(u)
			// if u is already present in the settled set.
			settled.add(u);

			e_Neighbours(u);
		}
	}

	// Method 2
	// To process all the neighbours
	// of the passed node
	private void e_Neighbours(int u)
	{

		int edgeDistance = -1;
		int newDistance = -1;

		// All the neighbors of v
		for (int i = 0; i < adj.get(u).size(); i++) {
			Node v = adj.get(u).get(i);

			// If current node hasn't already been processed
			if (!settled.contains(v.node)) {
				edgeDistance = v.cost;
				newDistance = dist[u] + edgeDistance;

				// If new distance is cheaper in cost
				if (newDistance < dist[v.node])
					dist[v.node] = newDistance;

				// Add the current node to the queue
				pq.add(new Node(v.node, dist[v.node]));
			}
		}
	}

	// Main driver method
	public static void main(String arg[])
	{

		int V = 5;
		int source = 0;

		// Adjacency list representation of the
		// connected edges by declaring List class object
		// Declaring object of type List<Node>
		List<List<Node> > adj
			= new ArrayList<List<Node> >();

		// Initialize list for every node
		for (int i = 0; i < V; i++) {
			List<Node> item = new ArrayList<Node>();
			adj.add(item);
		}

		// Inputs for the GFG(dpq) graph
		adj.get(0).add(new Node(1, 9));
		adj.get(0).add(new Node(2, 6));
		adj.get(0).add(new Node(3, 5));
		adj.get(0).add(new Node(4, 3));

		adj.get(2).add(new Node(1, 2));
		adj.get(2).add(new Node(3, 4));

		// Calculating the single source shortest path
		GFG dpq = new GFG(V);
		dpq.dijkstra(adj, source);

		// Printing the shortest path to all the nodes
		// from the source node
		System.out.println("The shorted path from node :");

		for (int i = 0; i < dpq.dist.length; i++)
			System.out.println(source + " to " + i + " is "
							+ dpq.dist[i]);
	}
}

// Class 2
// Helper class implementing Comparator interface
// Representing a node in the graph
class Node implements Comparator<Node> {

	// Member variables of this class
	public int node;
	public int cost;

	// Constructors of this class

	// Constructor 1
	public Node() {}

	// Constructor 2
	public Node(int node, int cost)
	{

		// This keyword refers to current instance itself
		this.node = node;
		this.cost = cost;
	}

	// Method 1
	@Override public int compare(Node node1, Node node2)
	{

		if (node1.cost < node2.cost)
			return -1;

		if (node1.cost > node2.cost)
			return 1;

		return 0;
	}
}