added Dijkstra algorithm

Author: Amit-Verma5262

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

@@ -0,0 +1,45 @@
+import java.util.*;
+
+public class DijkstraShortestPath {
+
+    /**
+     * Finds the shortest path distances from the startNode to all other nodes in a weighted graph.
+     *
+     * @param n The number of nodes in the graph.
+     * @param adjList The adjacency list where each entry contains pairs of neighboring nodes and edge weights.
+     * @param startNode The starting node for the shortest path calculation.
+     * @return An array where each index i contains the shortest distance from startNode to node i.
+     */
+    public int[] shortestPath(int n, Map<Integer, List<int[]>> adjList, int startNode) {
+        int[] distances = new int[n];
+        Arrays.fill(distances, Integer.MAX_VALUE);
+        distances[startNode] = 0;
+
+        PriorityQueue<int[]> minHeap = new PriorityQueue<>(Comparator.comparingInt(a -> a[1]));
+        minHeap.offer(new int[]{startNode, 0});
+
+        while (!minHeap.isEmpty()) {
+            int[] current = minHeap.poll();
+            int currentNode = current[0];
+            int currentDistance = current[1];
+
+            if (currentDistance > distances[currentNode]) {
+                continue;
+            }
+
+            List<int[]> neighbors = adjList.getOrDefault(currentNode, new ArrayList<>());
+            for (int[] neighbor : neighbors) {
+                int nextNode = neighbor[0];
+                int edgeWeight = neighbor[1];
+
+                int newDistance = distances[currentNode] + edgeWeight;
+                if (newDistance < distances[nextNode]) {
+                    distances[nextNode] = newDistance;
+                    minHeap.offer(new int[]{nextNode, newDistance});
+                }
+            }
+        }
+
+        return distances;
+    }
+}

src/test/java/com/thealgorithms/graph/DijkstraShortestPathTest.java

@@ -0,0 +1,63 @@
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+
+import java.util.HashMap;
+import java.util.List;
+import java.util.ArrayList;
+import java.util.Map;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+public class DijkstraShortestPathTest {
+
+    private DijkstraShortestPath dijkstra;
+
+    @BeforeEach
+    public void setUp() {
+        dijkstra = new DijkstraShortestPath();
+    }
+
+    @Test
+    public void testSinglePath() {
+        // Simple graph where the path is straightforward
+        Map<Integer, List<int[]>> adjList = new HashMap<>();
+        adjList.put(0, List.of(new int[]{1, 1}));
+        adjList.put(1, List.of(new int[]{2, 1}));
+        adjList.put(2, List.of(new int[]{3, 1}));
+        adjList.put(3, new ArrayList<>());
+
+        int[] result = dijkstra.shortestPath(4, adjList, 0);
+
+        int[] expected = {0, 1, 2, 3};
+        assertArrayEquals(expected, result, "Shortest path distances should match.");
+    }
+
+    @Test
+    public void testDisconnectedGraph() {
+        // Graph where some nodes are unreachable
+        Map<Integer, List<int[]>> adjList = new HashMap<>();
+        adjList.put(0, List.of(new int[]{1, 2}));
+        adjList.put(1, List.of(new int[]{2, 2}));
+        adjList.put(2, new ArrayList<>());
+        adjList.put(3, new ArrayList<>());
+
+        int[] result = dijkstra.shortestPath(4, adjList, 0);
+
+        int[] expected = {0, 2, 4, Integer.MAX_VALUE}; // node 3 is disconnected
+        assertArrayEquals(expected, result, "Shortest path should indicate unreachable nodes.");
+    }
+
+    @Test
+    public void testComplexGraph() {
+        // Complex graph with multiple paths
+        Map<Integer, List<int[]>> adjList = new HashMap<>();
+        adjList.put(0, List.of(new int[]{1, 4}, new int[]{2, 1}));
+        adjList.put(1, List.of(new int[]{3, 1}));
+        adjList.put(2, List.of(new int[]{1, 2}, new int[]{3, 5}));
+        adjList.put(3, new ArrayList<>());
+
+        int[] result = dijkstra.shortestPath(4, adjList, 0);
+
+        int[] expected = {0, 3, 1, 4};
+        assertArrayEquals(expected, result, "Distances should match expected shortest path distances.");
+    }
+}