LinkedListCycle.java

package linked_list;

import java.util.HashSet;
import java.util.Set;

/**
 * Created by gouthamvidyapradhan on 23/02/2017.
 Given a linked list, determine if it has a cycle in it.

 Follow up:
 Can you solve it without using extra space?
 */
public class LinkedListCycle
{
    private static Set<Integer> hashCode = new HashSet<>();
    static class ListNode
    {
        int val;
        ListNode next;
        ListNode(int x)
        {
            val = x;
            next = null;
        }
    }

    /**
     * Main method
     * @param args
     */
    public static void main(String[] args) throws Exception
    {
        ListNode node1 = new ListNode(1);
        ListNode node2 = new ListNode(2);
        ListNode node3 = new ListNode(3);
        node1.next = node2;
        node2.next = node3;
        node3.next = node1;
        System.out.println(new LinkedListCycle().hasCycle(node1));
    }

    public boolean hasCycle(ListNode head)
    {
        ListNode slow = head;
        ListNode fast = head;
        while(slow != null && fast != null)
        {
            slow = slow.next;
            fast = fast.next;
            if(fast != null)
                fast = fast.next;
            else break;
            if(fast != null && slow != null)
                if(fast.equals(slow)) return true;
        }
        return false;
    }

}