detects cycle in undirected Graph

Author: NirmalSilwal

GeeksForGeeks/Graph/DetectCycleUndirectedGraph.java

@@ -0,0 +1,95 @@
+package Graph;
+
+import java.util.ArrayList;
+
+public class DetectCycleUndirectedGraph {
+
+	private static boolean hasCycle(ArrayList<ArrayList<Integer>> adjList) {
+		return depthFirstTraverse(adjList);
+	}
+
+	private static boolean depthFirstTraverse(ArrayList<ArrayList<Integer>> adjList) {
+
+		boolean[] visited = new boolean[adjList.size()];
+
+		for (int i = 0; i < adjList.size(); i++) {
+
+			if (visited[i] == false) {
+
+				int isParent = -1;
+				if (dfsRecursion(adjList, i, visited, isParent) == true) {
+					return true;
+				}
+			}
+		}
+		return false;
+	}
+
+	private static boolean dfsRecursion(ArrayList<ArrayList<Integer>> adjList, int src, boolean[] visited, int parent) {
+		visited[src] = true;
+
+		for (Integer neighbor : adjList.get(src)) {
+
+			if (visited[neighbor] == false) {
+
+				if (dfsRecursion(adjList, neighbor, visited, src) == true) {
+					return true; // cycle exists
+				}
+			} else if (neighbor != parent)
+				return true;
+		}
+		return false;
+	}
+
+	public static void main(String[] args) {
+
+		// int totalVertices = 6; // test graph 1
+		// int totalVertices = 5; // test graph 2
+		int totalVertices = 4; // test graph 3
+
+		ArrayList<ArrayList<Integer>> adjList = new ArrayList<ArrayList<Integer>>(totalVertices);
+		for (int i = 0; i < totalVertices; i++) {
+			adjList.add(new ArrayList<Integer>());
+		}
+
+		// test graph 1, cycle - true
+
+		// addEdge(adjList, 0, 1);
+		// addEdge(adjList, 1, 2);
+		// addEdge(adjList, 1, 3);
+		// addEdge(adjList, 3, 2);
+		// addEdge(adjList, 2, 4);
+		// addEdge(adjList, 4, 5);
+
+		// test graph 2, cycle - false
+
+		// addEdge(adjList, 0, 1);
+		// addEdge(adjList, 1, 2);
+		// addEdge(adjList, 1, 4);
+		// addEdge(adjList, 2, 3);
+
+		// test graph 3, cycle - true
+
+		addEdge(adjList, 0, 1);
+		addEdge(adjList, 0, 3);
+		addEdge(adjList, 1, 3);
+		addEdge(adjList, 1, 2);
+		addEdge(adjList, 2, 3);
+
+		System.out.println("adjacency list representation of Graph\n");
+		display(adjList);
+
+		System.out.println("\nhas cycle: " + hasCycle(adjList)); // 2
+	}
+
+	private static void addEdge(ArrayList<ArrayList<Integer>> adjList, int u, int v) {
+		adjList.get(u).add(v);
+		adjList.get(v).add(u);
+	}
+
+	public static void display(ArrayList<ArrayList<Integer>> adjList) {
+		for (int i = 0; i < adjList.size(); i++) {
+			System.out.println(i + ": " + adjList.get(i));
+		}
+	}
+}