Create DetectCycleInDirectedGraph.java

Find cycles in directed graph

Author: joydeep1982

DetectCycleInDirectedGraph.java

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+package ds.algo.solution;
+
+import java.util.HashSet;
+import java.util.Set;
+
+public class DetectCycleInDirectedGraph {
+
+	public static void main(String[] args) {
+		Node one = new Node(1);
+		Node two = new Node(2);
+		Node three = new Node(3);
+		Node four = new Node(4);
+		Node five = new Node(5);
+		Node six = new Node(6);
+		
+		one.addChild(two).addChild(three).addChild(four);
+		two.addChild(three);
+		four.addChild(five);
+		five.addChild(six);
+		six.addChild(four);// this creates the cycle in the graph
+		
+		Set<Node> nodes = new HashSet<>();
+		nodes.add(one);
+		nodes.add(two);
+		nodes.add(three);
+		nodes.add(four);
+		nodes.add(five);
+		nodes.add(six);
+		
+		System.out.println(new DetectCycleInDirectedGraph().isCyclePresent(nodes));
+	}
+	
+	public boolean isCyclePresent(Set<Node> nodes) {
+		
+		Set<Node> whiteSet = new HashSet<>(nodes);
+		Set<Node> greySet = new HashSet<>();
+		Set<Node> blackSet = new HashSet<>();
+		
+		while (!whiteSet.isEmpty()) {
+			if(inspectNode(whiteSet.iterator().next(), whiteSet, greySet, blackSet)) {
+				return true;
+			}
+		}
+		
+		return false;
+	}
+	
+	private boolean inspectNode(Node n, Set<Node> whiteSet, Set<Node> greaySet, Set<Node> blackSet) {
+		
+		move(n, whiteSet, greaySet);
+		
+		for (Node node : n.getChildren()) {
+			if (blackSet.contains(node)) {
+				continue;
+			}
+			if (greaySet.contains(node)) {
+				return true;
+			}
+			if (inspectNode(node, whiteSet, greaySet, blackSet)) {
+				return true;
+			}
+		}
+		move(n, greaySet, blackSet);
+		
+		return false;
+	}
+	
+	private void move(Node n, Set<Node> source, Set<Node> destination) {
+		source.remove(n);
+		destination.add(n);
+	}
+}
+
+class Node {
+	
+	int value;
+	
+	Set<Node> children;
+	
+	Node(int val) {
+		this.children = new HashSet<Node>();
+		this.value = val;
+	}
+	
+	public Node addChild(Node n) {
+		children.add(n);
+		return this;
+	}
+	
+	public Set<Node> getChildren(){
+		return children;
+	}
+	
+	@Override
+	public String toString() {
+		return String.valueOf(this.value);
+	}
+}