Added Dijkstra Graph Algorithm

Author: Anshul-9923

src/main/java/com/thealgorithms/graph/DijkstraAlgorithm.java

@@ -0,0 +1,69 @@
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.HashMap;
+import java.util.List;
+import java.util.PriorityQueue;
+
+/**
+ * Finds the shortest paths from a source node to all other nodes in a graph using Dijkstra's algorithm.
+ *
+ * @param adjList The adjacency list representation of the graph where each edge has a weight.
+ * @param n The number of nodes in the graph.
+ * @param source The starting node for finding the shortest path.
+ * @return An array where the value at each index represents the shortest distance from the source to that node.
+ */
+public class DijkstraAlgorithm {
+
+    static class Pair {
+        int node;
+        int weight;
+
+        Pair(int node, int weight) {
+            this.node = node;
+            this.weight = weight;
+        }
+    }
+
+    public int[] dijkstra(HashMap<Integer, List<Pair>> adjList, int n, int source) {
+        int[] distances = new int[n];
+        Arrays.fill(distances, Integer.MAX_VALUE);
+        distances[source] = 0;
+
+        PriorityQueue<Pair> pq = new PriorityQueue<>((a, b) -> a.weight - b.weight);
+        pq.add(new Pair(source, 0));
+
+        while (!pq.isEmpty()) {
+            Pair current = pq.poll();
+            int currentNode = current.node;
+            int currentWeight = current.weight;
+
+            List<Pair> neighbors = adjList.get(currentNode);
+            if (neighbors != null) {
+                for (Pair neighbor : neighbors) {
+                    int newDist = currentWeight + neighbor.weight;
+                    if (newDist < distances[neighbor.node]) {
+                        distances[neighbor.node] = newDist;
+                        pq.add(new Pair(neighbor.node, newDist));
+                    }
+                }
+            }
+        }
+        return distances;
+    }
+
+    public static void main(String[] args) {
+        HashMap<Integer, List<Pair>> adjList = new HashMap<>();
+        adjList.put(0, Arrays.asList(new Pair(1, 4), new Pair(2, 1)));
+        adjList.put(1, Arrays.asList(new Pair(3, 1)));
+        adjList.put(2, Arrays.asList(new Pair(1, 2), new Pair(3, 5)));
+        adjList.put(3, new ArrayList<>());
+
+        DijkstraAlgorithm dijkstra = new DijkstraAlgorithm();
+        int[] distances = dijkstra.dijkstra(adjList, 4, 0);
+
+        System.out.println("Shortest distances from source 0:");
+        for (int i = 0; i < distances.length; i++) {
+            System.out.println("To node " + i + ": " + distances[i]);
+        }
+    }
+}