Added Java Implementation of Dijkstras Algorithm for Single Source Shortest Path (#187)

* Added Java Implementation of Dijkstras Algorithm for Single Source Shortest Path

* Added changes to Dijkstras Algorithm Implementation according to review

* Corrected spelling mistakes

graphs/README.md

@@ -5,3 +5,7 @@
 1. [Kruskal Algorithm](c-or-cpp/kruskal-algorithm.cpp)
 2. [Bellman Ford Algorithm](c-or-cpp/bellman-ford.cpp)
 3. [Prim's Algorithm](c-or-cpp/Prim's-algorithm.c)
+
+### Java
+
+1. [Dijkstras Algorithm](java/Dijkstras.java)

graphs/java/Dijkstras.java

@@ -0,0 +1,133 @@
+import java.util.*;
+
+class AdjListNode
+{
+	int dest;
+	int weight;
+	AdjListNode(int dest,int weight)
+	{
+		this.dest=dest;
+		this.weight=weight;
+	}
+}
+
+class NodeComparator implements Comparator<AdjListNode>
+{
+	@Override
+	public int compare(AdjListNode node1, AdjListNode node2) 
+	{		
+		if (node1.weight < node2.weight) 
+            		return -1; 
+        	if (node1.weight > node2.weight) 
+            		return 1; 
+        	return 0; 
+	}
+}
+
+class Dijkstras
+{
+	public int[] dijkstras(List<List<AdjListNode>> adj_list,int source)
+	{
+		int[] distance = new int[adj_list.size()];
+		Set<Integer> completed = new HashSet<Integer>();
+		for(int i=0;i<distance.length;++i)
+		{
+			distance[i] = Integer.MAX_VALUE;
+		}
+		PriorityQueue<AdjListNode> pq = new PriorityQueue<>(adj_list.size(),new NodeComparator());
+		AdjListNode source_node = new AdjListNode(source,0);
+		for(int i=0;i<distance.length;++i)
+		{
+			pq.add(new AdjListNode(i,Integer.MAX_VALUE));
+			
+		}
+		pq.add(source_node);
+		distance[source]=0;
+		while(!pq.isEmpty())
+		{
+			AdjListNode current = pq.remove();
+			if(!completed.contains(current.dest))
+			{
+				completed.add(current.dest);
+				for(AdjListNode n :adj_list.get(current.dest) )
+				{
+					if( distance[n.dest] > (distance[current.dest]+n.weight))
+					{
+						distance[n.dest] = distance[current.dest]+n.weight;
+						pq.add(new AdjListNode(n.dest, distance[n.dest]));
+					}
+				}
+			}
+		}
+		return distance;
+	}
+
+	public static void main(String args[])
+	{
+		/*
+		  	SAMPLE INPUT AND OUTPUT
+			___________________________
+
+			Input:
+			__________
+			Please enter the number of nodes N. Vertices will be [0,N-1]
+			6
+			Please Enter the number of Edges
+			9
+			Please enter each edge in the sequence <starting node> <destination node> <weight>
+			0 1 1
+			0 2 5
+			1 2 2
+			1 4 1
+			1 3 2
+			2 4 2
+			3 5 1
+			3 4 3
+			4 5 2
+			Please enter source vertex
+			0
+
+			Output:
+			___________
+			Distances from source 0
+			Node0--->0
+			Node1--->1
+			Node2--->3
+			Node3--->3
+			Node4--->2
+			Node5--->4
+		 */
+
+		List<List<AdjListNode>> adj_list = new ArrayList<List<AdjListNode>>();
+		System.out.println("Please enter the number of nodes N. Vertices will be [0,N-1]");
+		Scanner sc = new Scanner(System.in);
+		int v = sc.nextInt();
+		for(int i=0;i<v;++i)
+			adj_list.add(new ArrayList<AdjListNode>());
+		
+		System.out.println("Please enter the number of edges");
+		int e = sc.nextInt();
+		System.out.println("Please enter each edge in the sequence <starting node> <destination node> <weight>");
+		// Sample Data: 0 2 5 (edge from 0 to 2 with weight 5)
+		for(int i=0;i<e;++i)
+		{
+			int startnode = sc.nextInt();
+			int destnode = sc.nextInt();
+			int weight = sc.nextInt();
+		       	adj_list.get(startnode).add(new AdjListNode(destnode,weight));
+		}
+		int source;
+		System.out.println("Please enter source vertex");
+		source = sc.nextInt();
+		Dijkstras d = new Dijkstras();
+		int[] distances  = d.dijkstras(adj_list, source);  //source vertex is taken as 0
+		System.out.println("Distances from source "+source);
+		for(int i=0;i<distances.length;++i)
+		{
+			if(distances[i]==Integer.MAX_VALUE)
+				System.out.println("Node"+i+"--->"+"infinity");
+			else
+				System.out.println("Node"+i+"--->"+distances[i]);
+		}
+	}
+}