Merge pull request #418 from guptushar27/patch-4

Create DijkstraAlgorithm2.java

Author: Pradeepsingh61

Java/algorithms/graph_algorithms/DijkstraAlgorithm2.java

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+
+import java.util.*;
+
+/**
+ * Algorithm Name : Dijkstra Algorithm
+ * Programming Language: Java
+ * Category: Graph Algorithms
+ * Difficulty Level: Medium (Intermediate)
+ *
+ * Problem Statement:
+ * Implement Dijkstra’s Algorithm to find the shortest path from a single
+ * source vertex to all other vertices in a weighted graph (non-negative edge weights)
+ * using a priority queue (min-heap) for optimization.
+ *
+ * Requirements:
+ * - Input:
+ *      Number of vertices: V
+ *      Number of edges: E
+ *      Edges with weights (u, v, w)
+ *      Starting vertex: s
+ * - Output:
+ *      Shortest distance from source to all vertices
+ *
+ * Complexity:
+ * - Time Complexity: O((V + E) * log V)
+ * - Space Complexity: O(V + E)
+ *
+ * Additional Notes:
+ * - Negative edge weights are not supported (Dijkstra assumes non-negative edges).
+ */
+public class DijkstraAlgorithm2 {
+
+    // Inner class to represent a graph edge (destination vertex and weight)
+    static class Edge {
+        int vertex, weight;
+        Edge(int vertex, int weight) {
+            this.vertex = vertex;
+            this.weight = weight;
+        }
+    }
+
+    /**
+     * Dijkstra’s Algorithm implementation.
+     * Uses a PriorityQueue (min-heap) to efficiently get the vertex with the smallest distance.
+     */
+    public static int[] dijkstra(List<List<Edge>> graph, int source, int V) {
+        // Initialize distance array with "infinity" (here, Integer.MAX_VALUE)
+        int[] dist = new int[V];
+        Arrays.fill(dist, Integer.MAX_VALUE);
+        dist[source] = 0;
+
+        // Min-heap: stores pairs (distance, vertex)
+        PriorityQueue<int[]> pq = new PriorityQueue<>(Comparator.comparingInt(a -> a[0]));
+        pq.offer(new int[]{0, source}); // distance = 0, vertex = source
+
+        while (!pq.isEmpty()) {
+            int[] current = pq.poll();
+            int currDist = current[0];
+            int u = current[1];
+
+            // Skip if we have already found a better distance
+            if (currDist > dist[u]) continue;
+
+            // Traverse all adjacent vertices
+            for (Edge edge : graph.get(u)) {
+                int v = edge.vertex;
+                int weight = edge.weight;
+
+                // Relaxation step: if found shorter path to v, update it
+                if (dist[u] + weight < dist[v]) {
+                    dist[v] = dist[u] + weight;
+                    pq.offer(new int[]{dist[v], v});
+                }
+            }
+        }
+        return dist;
+    }
+
+    /**
+     * Utility function to add an edge to the adjacency list.
+     * Since this example is for a directed graph — remove one addEdge call below for undirected.
+     */
+    public static void addEdge(List<List<Edge>> graph, int u, int v, int w) {
+        graph.get(u).add(new Edge(v, w));
+    }
+
+    /**
+     * Sample test cases:
+     * - Small connected graph
+     * - Graph with multiple paths
+     * - Single-node graph
+     */
+    public static void main(String[] args) {
+        // Example 1: Small connected graph
+        int V1 = 5;
+        List<List<Edge>> graph1 = new ArrayList<>();
+        for (int i = 0; i < V1; i++) graph1.add(new ArrayList<>());
+
+        addEdge(graph1, 0, 1, 10);
+        addEdge(graph1, 0, 4, 5);
+        addEdge(graph1, 1, 2, 1);
+        addEdge(graph1, 4, 1, 3);
+        addEdge(graph1, 4, 2, 9);
+        addEdge(graph1, 2, 3, 4);
+        addEdge(graph1, 4, 3, 2);
+
+        int source1 = 0;
+        int[] dist1 = dijkstra(graph1, source1, V1);
+        System.out.println("Shortest distances from vertex " + source1 + ": " + Arrays.toString(dist1));
+        System.out.println();
+
+        // Example 2: Graph with multiple paths
+        int V2 = 4;
+        List<List<Edge>> graph2 = new ArrayList<>();
+        for (int i = 0; i < V2; i++) graph2.add(new ArrayList<>());
+
+        addEdge(graph2, 0, 1, 1);
+        addEdge(graph2, 0, 2, 4);
+        addEdge(graph2, 1, 2, 2);
+        addEdge(graph2, 1, 3, 6);
+        addEdge(graph2, 2, 3, 3);
+
+        int source2 = 0;
+        int[] dist2 = dijkstra(graph2, source2, V2);
+        System.out.println("Shortest distances from vertex " + source2 + ": " + Arrays.toString(dist2));
+        System.out.println();
+
+        // Example 3: Single-node graph
+        int V3 = 1;
+        List<List<Edge>> graph3 = new ArrayList<>();
+        for (int i = 0; i < V3; i++) graph3.add(new ArrayList<>());
+
+        int source3 = 0;
+        int[] dist3 = dijkstra(graph3, source3, V3);
+        System.out.println("Shortest distances from vertex " + source3 + ": " + Arrays.toString(dist3));
+    }
+}