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Cycle detection using java in linked lists

Author: inimitable034

Data Structures/Linked Lists/Singly Linked List/cycle-detection-using-java.java.txt

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+// Java program to detect cycle/ loop in a linked list
+
+import java.util.*;
+
+public class LinkedList {
+
+	static Node head; // head of list
+
+	/* Linked list Node*/
+	static class Node {
+		int data;
+		Node next;
+		Node(int d)
+		{
+			data = d;
+			next = null;
+		}
+	}
+
+	/* Inserts a new Node at front of the list. */
+
+	static public void push(int new_data)
+	{
+		/* 1 & 2: Allocate the Node &
+				Put in the data*/
+		Node new_node = new Node(new_data);
+
+		/* 3. Make next of new Node as head */
+		new_node.next = head;
+
+		/* 4. Move the head to point to new Node */
+		head = new_node;
+	}
+
+	// Returns true if there is a loop in linked
+	// list else returns false.
+
+	static boolean detectLoop(Node h)
+	{
+		HashSet<Node> s = new HashSet<Node>();
+		while (h != null) {
+
+			// If we have already has this node
+			// in hashmap it means their is a cycle
+			// (Because you we encountering the
+			// node second time).
+
+			if (s.contains(h))
+				return true;
+
+			// If we are seeing the node for
+			// the first time, insert it in hash
+
+			s.add(h);
+
+			h = h.next;
+		}
+
+		return false;
+	}
+
+	
+	public static void main(String[] args)
+	{
+		LinkedList llist = new LinkedList();
+
+		llist.push(20);
+		llist.push(4);
+		llist.push(15);
+		llist.push(10);
+
+		/*Create loop for testing */
+
+		llist.head.next.next.next.next = llist.head;
+
+		if (detectLoop(head))
+			System.out.println("Loop found");
+		else
+			System.out.println("No Loop");
+	}
+}
+
+