Dijkstra showcase, weighted graph implementation

Author: ZapDos7

README.md

@@ -134,6 +134,8 @@
 ### Graphs
 - [x] [Terminology and Representations of Graphs](http://www.techiedelight.com/terminology-and-representations-of-graphs/)
 - [x] Given a list of edges and tasked to build your own graph from the edges
+- [x] Implement Dijkstra’s algorithm (implemented Weighted Graph to showcase this)
+
 ### Matrix
 
 ### Trees
@@ -268,7 +270,6 @@
 
 
 - Graphs
-    - [ ] Implement Dijkstra’s algorithm
     - [ ] Implement Topological sort
     - [ ] Implement Bellman-Ford algorithm
     - [ ] Implement Floyd-Warshall algorithm

src/dataStructures/graphs/WeightedGraph.java

@@ -0,0 +1,86 @@
+package dataStructures.graphs;
+
+import java.util.*;
+
+import dataStructures.graphs.CustomGraph.Vertex;
+
+public class WeightedGraph {
+    private Map<Vertex, List<Edge>> adjVertices;
+
+    public static class Edge {
+        Vertex destination;
+        int weight;
+
+        public Edge(Vertex destination, int weight) {
+            this.destination = destination;
+            this.weight = weight;
+        }
+
+        @Override
+        public String toString() {
+            return "Edge{to=" + destination + ", weight=" + weight + '}';
+        }
+    }
+
+    public Map<Vertex, Integer> dijkstra(Vertex source) {
+        Map<Vertex, Integer> distances = new HashMap<>();
+        Map<Vertex, Vertex> previous = new HashMap<>();
+        PriorityQueue<VertexDistance> pq = new PriorityQueue<>(Comparator.comparingInt(vd -> vd.distance));
+
+        for (Vertex vertex : adjVertices.keySet()) {
+            distances.put(vertex, Integer.MAX_VALUE);
+        }
+        distances.put(source, 0);
+        pq.add(new VertexDistance(source, 0));
+
+        while (!pq.isEmpty()) {
+            VertexDistance current = pq.poll();
+            Vertex u = current.vertex;
+
+            for (Edge edge : adjVertices.getOrDefault(u, Collections.emptyList())) {
+                Vertex v = edge.destination;
+                int newDist = distances.get(u) + edge.weight;
+                if (newDist < distances.get(v)) {
+                    distances.put(v, newDist);
+                    previous.put(v, u);
+                    pq.add(new VertexDistance(v, newDist));
+                }
+            }
+        }
+
+        return distances;
+    }
+
+    private static class VertexDistance {
+        Vertex vertex;
+        int distance;
+
+        public VertexDistance(Vertex vertex, int distance) {
+            this.vertex = vertex;
+            this.distance = distance;
+        }
+    }
+
+    // demo
+
+    public static void main(String[] args) {
+        Vertex a = new Vertex("A");
+        Vertex b = new Vertex("B");
+        Vertex c = new Vertex("C");
+        Vertex d = new Vertex("D");
+
+        WeightedGraph graph = new WeightedGraph();
+        graph.adjVertices = new HashMap<>();
+
+        graph.adjVertices.put(a, Arrays.asList(new Edge(b, 1), new Edge(c, 4)));
+        graph.adjVertices.put(b, Arrays.asList(new Edge(c, 2), new Edge(d, 5)));
+        graph.adjVertices.put(c, Arrays.asList(new Edge(d, 1)));
+        graph.adjVertices.put(d, new ArrayList<>());
+
+        Map<Vertex, Integer> distances = graph.dijkstra(a);
+        for (Map.Entry<Vertex, Integer> entry : distances.entrySet()) {
+            System.out.println("Distance from A to " + entry.getKey().label + " = " + entry.getValue());
+        }
+    }
+
+}