fix: Add more test cases to improve code coverage

Author: sairamsharan

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

@@ -1,16 +1,14 @@
 package com.thealgorithms.graph;
 
-import static org.junit.jupiter.api.Assertions.assertEquals;
-import static org.junit.jupiter.api.Assertions.assertTrue;
-
+import static org.junit.jupiter.api.Assertions.*;
 import java.util.*;
 import org.junit.jupiter.api.Test;
 
 public class HierholzerAlgorithmTest {
 
     @Test
     public void testFindsEulerianCircuitInSimpleTriangleGraph() {
-        // Create a simple triangle graph where a circuit definitely exists: 0-1, 1-2, 2-0
+        // A simple triangle graph where a circuit definitely exists: 0-1, 1-2, 2-0
         Map<Integer, LinkedList<Integer>> graph = new HashMap<>();
         graph.put(0, new LinkedList<>(Arrays.asList(1, 2)));
         graph.put(1, new LinkedList<>(Arrays.asList(0, 2)));
@@ -31,16 +29,42 @@ public void testFindsEulerianCircuitInSimpleTriangleGraph() {
     }
 
     @Test
-    public void testHandlesGraphWithNoEulerianCircuit() {
-        // Create a graph where a vertex has an odd degree
+    public void testFailsForGraphWithOddDegreeVertices() {
+        // Create a graph where vertices 0 and 1 have an odd degree (1)
         Map<Integer, LinkedList<Integer>> graph = new HashMap<>();
         graph.put(0, new LinkedList<>(Collections.singletonList(1)));
         graph.put(1, new LinkedList<>(Collections.singletonList(0)));
-        graph.put(2, new LinkedList<>(Collections.emptyList())); // Vertex 2 is isolated
 
         HierholzerAlgorithm algorithm = new HierholzerAlgorithm(graph);
 
         // The algorithm should correctly identify that no circuit exists
-        assertEquals(false, algorithm.hasEulerianCircuit());
+        assertFalse(algorithm.hasEulerianCircuit());
+        // The find method should return an empty list
+        assertTrue(algorithm.findEulerianCircuit().isEmpty());
+    }
+    
+    @Test
+    public void testFailsForDisconnectedGraph() {
+        // Create a graph with two separate triangles (0-1-2 and 3-4-5)
+        Map<Integer, LinkedList<Integer>> graph = new HashMap<>();
+        graph.put(0, new LinkedList<>(Arrays.asList(1, 2)));
+        graph.put(1, new LinkedList<>(Arrays.asList(0, 2)));
+        graph.put(2, new LinkedList<>(Arrays.asList(0, 1)));
+        graph.put(3, new LinkedList<>(Arrays.asList(4, 5)));
+        graph.put(4, new LinkedList<>(Arrays.asList(3, 5)));
+        graph.put(5, new LinkedList<>(Arrays.asList(3, 4)));
+
+        HierholzerAlgorithm algorithm = new HierholzerAlgorithm(graph);
+
+        // All degrees are even, but the graph is not connected, so no circuit exists
+        assertFalse(algorithm.hasEulerianCircuit());
+    }
+
+    @Test
+    public void testHandlesEmptyGraph() {
+        Map<Integer, LinkedList<Integer>> graph = new HashMap<>();
+        HierholzerAlgorithm algorithm = new HierholzerAlgorithm(graph);
+        assertTrue(algorithm.hasEulerianCircuit());
+        assertTrue(algorithm.findEulerianCircuit().isEmpty());
     }
-}
+}