Bael 2950 graph has cycle check for directed graphs (#7088)

* BAEL-2950 : Graph Check If Cycle Exists

* BAEL-2950 : Minor Refactoring

* BAEL-2950 Cycle detection

* BAEL-2950 : Renaming unit test file

algorithms-miscellaneous-3/src/main/java/com/baeldung/algorithms/graphcycledetection/domain/Graph.java

@@ -0,0 +1,51 @@
+package com.baeldung.algorithms.graphcycledetection.domain;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class Graph {
+
+    private List<Vertex> vertices;
+
+    public Graph() {
+        this.vertices = new ArrayList<>();
+    }
+
+    public Graph(List<Vertex> vertices) {
+        this.vertices = vertices;
+    }
+
+    public void addVertex(Vertex vertex) {
+        this.vertices.add(vertex);
+    }
+
+    public void addEdge(Vertex from, Vertex to) {
+        from.addNeighbour(to);
+    }
+
+    public boolean hasCycle() {
+        for (Vertex vertex : vertices) {
+            if (!vertex.isVisited() && hasCycle(vertex)) {
+                return true;
+            }
+        }
+        return false;
+    }
+
+    public boolean hasCycle(Vertex sourceVertex) {
+        sourceVertex.setBeingVisited(true);
+
+        for (Vertex neighbour : sourceVertex.getAdjacencyList()) {
+            if (neighbour.isBeingVisited()) {
+                // backward edge exists
+                return true;
+            } else if (!neighbour.isVisited() && hasCycle(neighbour)) {
+                return true;
+            }
+        }
+
+        sourceVertex.setBeingVisited(false);
+        sourceVertex.setVisited(true);
+        return false;
+    }
+}

algorithms-miscellaneous-3/src/main/java/com/baeldung/algorithms/graphcycledetection/domain/Vertex.java

@@ -0,0 +1,56 @@
+package com.baeldung.algorithms.graphcycledetection.domain;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class Vertex {
+
+    private String label;
+
+    private boolean visited;
+
+    private boolean beingVisited;
+
+    private List<Vertex> adjacencyList;
+
+    public Vertex(String label) {
+        this.label = label;
+        this.adjacencyList = new ArrayList<>();
+    }
+
+    public String getLabel() {
+        return label;
+    }
+
+    public void setLabel(String label) {
+        this.label = label;
+    }
+
+    public boolean isVisited() {
+        return visited;
+    }
+
+    public void setVisited(boolean visited) {
+        this.visited = visited;
+    }
+
+    public boolean isBeingVisited() {
+        return beingVisited;
+    }
+
+    public void setBeingVisited(boolean beingVisited) {
+        this.beingVisited = beingVisited;
+    }
+
+    public List<Vertex> getAdjacencyList() {
+        return adjacencyList;
+    }
+
+    public void setAdjacencyList(List<Vertex> adjacencyList) {
+        this.adjacencyList = adjacencyList;
+    }
+
+    public void addNeighbour(Vertex adjacent) {
+        this.adjacencyList.add(adjacent);
+    }
+}

algorithms-miscellaneous-3/src/test/java/com/baeldung/algorithms/graphcycledetection/GraphCycleDetectionUnitTest.java

@@ -0,0 +1,56 @@
+package com.baeldung.algorithms.graphcycledetection;
+
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+
+import org.junit.Test;
+
+import com.baeldung.algorithms.graphcycledetection.domain.Graph;
+import com.baeldung.algorithms.graphcycledetection.domain.Vertex;
+
+public class GraphCycleDetectionUnitTest {
+
+    @Test
+    public void givenGraph_whenCycleExists_thenReturnTrue() {
+
+        Vertex vertexA = new Vertex("A");
+        Vertex vertexB = new Vertex("B");
+        Vertex vertexC = new Vertex("C");
+        Vertex vertexD = new Vertex("D");
+
+        Graph graph = new Graph();
+        graph.addVertex(vertexA);
+        graph.addVertex(vertexB);
+        graph.addVertex(vertexC);
+        graph.addVertex(vertexD);
+
+        graph.addEdge(vertexA, vertexB);
+        graph.addEdge(vertexB, vertexC);
+        graph.addEdge(vertexC, vertexA);
+        graph.addEdge(vertexD, vertexC);
+
+        assertTrue(graph.hasCycle());
+    }
+
+    @Test
+    public void givenGraph_whenNoCycleExists_thenReturnFalse() {
+
+        Vertex vertexA = new Vertex("A");
+        Vertex vertexB = new Vertex("B");
+        Vertex vertexC = new Vertex("C");
+        Vertex vertexD = new Vertex("D");
+
+        Graph graph = new Graph();
+        graph.addVertex(vertexA);
+        graph.addVertex(vertexB);
+        graph.addVertex(vertexC);
+        graph.addVertex(vertexD);
+
+        graph.addEdge(vertexA, vertexB);
+        graph.addEdge(vertexB, vertexC);
+        graph.addEdge(vertexA, vertexC);
+        graph.addEdge(vertexD, vertexC);
+
+        assertFalse(graph.hasCycle());
+    }
+}