Breadth First Traversal of Graph, connected & disconnected graph, find no. of components

Author: NirmalSilwal

GeeksForGeeks/Graph/BFSTraversal.java

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+package Graph;
+
+import java.util.ArrayList;
+import java.util.LinkedList;
+
+public class BFSTraversal {
+
+	public static void main(String[] args) {
+
+		// int totalVertices = 5; // test graph 1
+		// int totalVertices = 7; // test graph 2
+		int totalVertices = 9; // test graph 3
+
+		ArrayList<ArrayList<Integer>> adjList = new ArrayList<ArrayList<Integer>>(totalVertices);
+		for (int i = 0; i < totalVertices; i++) {
+			adjList.add(new ArrayList<Integer>(i));
+		}
+
+		// note: be careful to adjust totalVertices per test cases of Graph
+
+		/* test graph 1 - connected graph */
+
+		// addEdge(adjList, 0, 1);
+		// addEdge(adjList, 0, 2);
+		// addEdge(adjList, 1, 2);
+		// addEdge(adjList, 1, 3);
+		// addEdge(adjList, 2, 3);
+		// addEdge(adjList, 2, 4);
+		// addEdge(adjList, 3, 4);
+
+		// int sourceVertex = 0;
+		// boolean[] visited = new boolean[totalVertices];
+		// breadthFirstSearchTraverse(adjList, sourceVertex, visited);
+		// System.out.println();
+
+		/* test graph 2 - disconnected graph with two components */
+
+		// addEdge(adjList, 0, 1);
+		// addEdge(adjList, 0, 2);
+		// addEdge(adjList, 1, 3);
+		// addEdge(adjList, 2, 3);
+		// addEdge(adjList, 4, 5);
+		// addEdge(adjList, 4, 6);
+		// addEdge(adjList, 5, 6);
+		//
+		// bfsDisconnectedGraph(adjList);
+
+		/* test graph 3 - disconnected graph with three components */
+
+		addEdge(adjList, 0, 1);
+		addEdge(adjList, 0, 2);
+		addEdge(adjList, 1, 2);
+		addEdge(adjList, 3, 4);
+		addEdge(adjList, 5, 6);
+		addEdge(adjList, 5, 7);
+		addEdge(adjList, 7, 8);
+
+		bfsDisconnectedGraph(adjList);
+	}
+
+	private static void addEdge(ArrayList<ArrayList<Integer>> adj, int u, int v) {
+		adj.get(u).add(v);
+		adj.get(v).add(u);
+	}
+
+	/*
+	 * Provided adjacency list representation of Graph, doing BFS traversal.
+	 * 
+	 * It can only deal with connected graph.
+	 */
+
+	// O(V+E) Time
+	public static void breadthFirstSearchTraverse(ArrayList<ArrayList<Integer>> adj, int src, boolean[] visited) {
+
+		LinkedList<Integer> queue = new LinkedList<>();
+
+		queue.addLast(src);
+		visited[src] = true;
+
+		while (!queue.isEmpty()) {
+			int removedVertex = queue.removeFirst();
+			System.out.print(removedVertex + " ");
+
+			for (Integer neighbor : adj.get(removedVertex)) {
+				if (visited[neighbor] == false) {
+					queue.addLast(neighbor);
+					visited[neighbor] = true;
+				}
+			}
+		}
+	}
+
+	/*
+	 * if the graph is either disconnected or connected
+	 * 
+	 * flag counts no. of islands in graph
+	 * 
+	 * O(V+E) Time, V is no. of Vertices, E is no. of Edges
+	 * 
+	 */
+	public static void bfsDisconnectedGraph(ArrayList<ArrayList<Integer>> adj) {
+		boolean[] visited = new boolean[adj.size()];
+		int flag = 0; // to count connected components
+		
+		for (int i = 0; i < adj.size(); i++) {
+			if (visited[i] == false) {
+				flag++;
+				breadthFirstSearchTraverse(adj, i, visited);
+			}
+		}
+		System.out.println("\n\nno. of connected components/ Island in Graph: " + flag);
+	}
+}