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Data Structures/Graphs/ConnectedComponent.java

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+import java.util.ArrayList;
+import java.util.HashSet;
+import java.util.Set;
+
+/**
+ * A class that counts the number of connected components in a graph
+ * 
+ * @author Lukas Keul
+ *
+ */
+class Graph<E extends Comparable<E>> {
+
+	class Node {
+		E name;
+
+		public Node(E name) {
+			this.name = name;
+		}
+	}
+
+	class Edge {
+		Node startNode, endNode;
+
+		public Edge(Node startNode, Node endNode) {
+			this.startNode = startNode;
+			this.endNode = endNode;
+		}
+	}
+
+	ArrayList<Edge> edgeList;
+	ArrayList<Node> nodeList;
+
+	public Graph() {
+		edgeList = new ArrayList<Edge>();
+		nodeList = new ArrayList<Node>();
+	}
+
+	/**
+	 * Adds a new Edge to the graph. If the nodes aren't yet in nodeList, they
+	 * will be added to it.
+	 * 
+	 * @param startNode
+	 *            the starting Node from the edge
+	 * 
+	 * @param endNode
+	 *            the ending Node from the edge
+	 */
+	public void addEdge(E startNode, E endNode) {
+		Node start = null, end = null;
+		for (Node node : nodeList) {
+			if (startNode.compareTo(node.name) == 0) {
+				start = node;
+			}
+			else if (endNode.compareTo(node.name) == 0) {
+				end = node;
+			}
+		}
+		if (start == null) {
+			start = new Node(startNode);
+			nodeList.add(start);
+		}
+		if (end == null) {
+			end = new Node(endNode);
+			nodeList.add(end);
+		}
+
+		edgeList.add(new Edge(start, end));
+	}
+
+	/**
+	 * Main method used for counting the connected components. Iterates through
+	 * the array of nodes to do a depth first search to get all nodes of the
+	 * graph from the actual node. These nodes are added to the array
+	 * markedNodes and will be ignored if they are chosen in the nodeList.
+	 * 
+	 * @return returns the amount of unconnected graphs
+	 * 
+	 */
+	public int countGraphs() {
+		int count = 0;
+		Set<Node> markedNodes = new HashSet<Node>();
+
+		for (Node n : nodeList) {
+			if (!markedNodes.contains(n)) {
+				markedNodes.add(n);
+				markedNodes.addAll(depthFirstSearch(n, new ArrayList<Node>()));
+				count++;
+			}
+		}
+
+		return count;
+	}
+
+	/**
+	 * Implementation of depth first search.
+	 * 
+	 * @param n
+	 *            the actual visiting node
+	 * 
+	 * @param visited
+	 *            A list of already visited nodes in the depth first search
+	 * 
+	 * @return returns a set of visited nodes
+	 *
+	 */
+	public ArrayList<Node> depthFirstSearch(Node n, ArrayList<Node> visited) {
+		visited.add(n);
+		for (Edge e : edgeList) {
+			if (e.startNode.equals(n) && !visited.contains(e.endNode)) {
+				depthFirstSearch(e.endNode, visited);
+			}
+		}
+		return visited;
+	}
+}
+
+public class ConnectedComponent {
+
+	public static void main(String[] args) {
+		Graph graphChars = new Graph();
+
+		// Graph 1
+		 graphChars.addEdge('a', 'b');
+		 graphChars.addEdge('a', 'e');
+		 graphChars.addEdge('b', 'e');
+		 graphChars.addEdge('b', 'c');
+		 graphChars.addEdge('c', 'd');
+		 graphChars.addEdge('d', 'a');
+		 
+		 graphChars.addEdge('x', 'y');
+		 graphChars.addEdge('x', 'z');
+		 
+		 graphChars.addEdge('w', 'w');
+
+		Graph graphInts = new Graph();
+		
+		// Graph 2
+		graphInts.addEdge(1, 2);
+		graphInts.addEdge(2, 3);
+		graphInts.addEdge(2, 4);
+		graphInts.addEdge(3, 5);
+		
+		graphInts.addEdge(7, 8);
+		graphInts.addEdge(8, 10);
+		graphInts.addEdge(10, 8);
+
+		System.out.println("Amount of different char-graphs: " + graphChars.countGraphs());
+		System.out.println("Amount of different int-graphs: " + graphInts.countGraphs());
+	}
+}