Hierholzer path algorithm (#6822)

* Added HierholzerEulerianPath algorithm * Added Hierholzer Algorith to find Eulerian Path * Added Hierholzer Algorith to find Eulerian Path * Added Hierholzer Algorith to find Eulerian Path * Added Hierholzer Algorith to find Eulerian Path * Added Hierholzer Algorith to find Eulerian Path * Added Hierholzer Algorith to find Eulerian Path * Added Hierholzer Algorith to find Eulerian Path --------- Co-authored-by: crashmovies <swarnakarharshendu420@gmail.com>
2025-12-19 07:00:35 +08:00 · 2025-10-18 03:23:26 +05:30
parent 4858ec9af0
commit 6b7d201657
2 changed files with 472 additions and 0 deletions
--- a/src/test/java/com/thealgorithms/graph/HierholzerEulerianPathTest.java
+++ b/src/test/java/com/thealgorithms/graph/HierholzerEulerianPathTest.java
@@ -0,0 +1,169 @@
+package com.thealgorithms.graph;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Unit tests for {@link HierholzerEulerianPath}.
+ *
+ * This test suite validates Hierholzer's Algorithm implementation
+ * for finding Eulerian Paths and Circuits in directed graphs.
+ *
+ * <p>Coverage includes:
+ * <ul>
+ *   <li>Basic Eulerian Circuit</li>
+ *   <li>Eulerian Path</li>
+ *   <li>Disconnected graphs</li>
+ *   <li>Single-node graphs</li>
+ *   <li>Graphs with no edges</li>
+ *   <li>Graphs that do not have any Eulerian Path/Circuit</li>
+ * </ul>
+ * </p>
+ */
+class HierholzerEulerianPathTest {
+
+    @Test
+    void testSimpleEulerianCircuit() {
+        HierholzerEulerianPath.Graph graph = new HierholzerEulerianPath.Graph(3);
+        graph.addEdge(0, 1);
+        graph.addEdge(1, 2);
+        graph.addEdge(2, 0);
+
+        HierholzerEulerianPath solver = new HierholzerEulerianPath(graph);
+        List<Integer> result = solver.findEulerianPath();
+
+        // Eulerian Circuit: [0, 1, 2, 0]
+        List<Integer> expected = Arrays.asList(0, 1, 2, 0);
+        assertEquals(expected, result);
+    }
+
+    @Test
+    void testEulerianPathDifferentStartEnd() {
+        HierholzerEulerianPath.Graph graph = new HierholzerEulerianPath.Graph(4);
+        graph.addEdge(0, 1);
+        graph.addEdge(1, 2);
+        graph.addEdge(2, 3);
+        graph.addEdge(3, 1);
+
+        HierholzerEulerianPath solver = new HierholzerEulerianPath(graph);
+        List<Integer> result = solver.findEulerianPath();
+
+        // Eulerian Path: [0, 1, 2, 3, 1]
+        List<Integer> expected = Arrays.asList(0, 1, 2, 3, 1);
+        assertEquals(expected, result);
+    }
+
+    @Test
+    void testNoEulerianPathExists() {
+        HierholzerEulerianPath.Graph graph = new HierholzerEulerianPath.Graph(3);
+        graph.addEdge(0, 1);
+        graph.addEdge(1, 2);
+        // Edge 2->0 missing, so it's not Eulerian Circuit
+
+        HierholzerEulerianPath solver = new HierholzerEulerianPath(graph);
+        List<Integer> result = solver.findEulerianPath();
+
+        assertEquals(result, Arrays.asList(0, 1, 2));
+    }
+
+    @Test
+    void testDisconnectedGraph() {
+        HierholzerEulerianPath.Graph graph = new HierholzerEulerianPath.Graph(4);
+        graph.addEdge(0, 1);
+        graph.addEdge(2, 3); // disconnected component
+
+        HierholzerEulerianPath solver = new HierholzerEulerianPath(graph);
+        List<Integer> result = solver.findEulerianPath();
+
+        // Disconnected graph cannot have an Eulerian path
+        assertTrue(result.isEmpty());
+    }
+
+    @Test
+    void testGraphWithSelfLoop() {
+        HierholzerEulerianPath.Graph graph = new HierholzerEulerianPath.Graph(3);
+        graph.addEdge(0, 0); // self loop
+        graph.addEdge(0, 1);
+        graph.addEdge(1, 2);
+        graph.addEdge(2, 0);
+
+        HierholzerEulerianPath solver = new HierholzerEulerianPath(graph);
+        List<Integer> result = solver.findEulerianPath();
+
+        // Eulerian Circuit with self-loop included: [0, 0, 1, 2, 0]
+        assertEquals(Arrays.asList(0, 0, 1, 2, 0), result);
+    }
+
+    @Test
+    void testSingleNodeNoEdges() {
+        HierholzerEulerianPath.Graph graph = new HierholzerEulerianPath.Graph(1);
+
+        HierholzerEulerianPath solver = new HierholzerEulerianPath(graph);
+        List<Integer> result = solver.findEulerianPath();
+
+        // Only one vertex and no edges
+        assertEquals(Collections.singletonList(0), result);
+    }
+
+    @Test
+    void testSingleNodeWithLoop() {
+        HierholzerEulerianPath.Graph graph = new HierholzerEulerianPath.Graph(1);
+        graph.addEdge(0, 0);
+
+        HierholzerEulerianPath solver = new HierholzerEulerianPath(graph);
+        List<Integer> result = solver.findEulerianPath();
+
+        // Eulerian circuit on a single node with a self-loop
+        assertEquals(Arrays.asList(0, 0), result);
+    }
+
+    @Test
+    void testComplexEulerianCircuit() {
+        HierholzerEulerianPath.Graph graph = new HierholzerEulerianPath.Graph(5);
+        graph.addEdge(0, 1);
+        graph.addEdge(1, 2);
+        graph.addEdge(2, 3);
+        graph.addEdge(3, 4);
+        graph.addEdge(4, 0);
+        graph.addEdge(1, 3);
+        graph.addEdge(3, 1);
+
+        HierholzerEulerianPath solver = new HierholzerEulerianPath(graph);
+        List<Integer> result = solver.findEulerianPath();
+
+        // Verify all edges are used
+        int totalEdges = 7;
+        assertEquals(totalEdges + 1, result.size(), "Path must contain all edges + 1 vertices");
+        assertEquals(result.get(0), result.get(result.size() - 1), "Must form a circuit");
+    }
+
+    @Test
+    void testMultipleEdgesBetweenSameNodes() {
+        HierholzerEulerianPath.Graph graph = new HierholzerEulerianPath.Graph(3);
+        graph.addEdge(0, 1);
+        graph.addEdge(0, 1);
+        graph.addEdge(1, 2);
+        graph.addEdge(2, 0);
+
+        HierholzerEulerianPath solver = new HierholzerEulerianPath(graph);
+        List<Integer> result = solver.findEulerianPath();
+
+        // Hava a Eulerian Path but not a Eulerian Circuit
+        assertEquals(result, Arrays.asList(0, 1, 2, 0, 1));
+    }
+
+    @Test
+    void testCoverageForEmptyGraph() {
+        HierholzerEulerianPath.Graph graph = new HierholzerEulerianPath.Graph(0);
+        HierholzerEulerianPath solver = new HierholzerEulerianPath(graph);
+        List<Integer> result = solver.findEulerianPath();
+
+        // Empty graph has no vertices or path
+        assertTrue(result.isEmpty());
+    }
+}