Given a linked list, determine if it has a cycle in it.

Follow up:

Can you solve it without using extra space?

 

public class Solution {

    public boolean hasCycle(ListNode head) {

        if (head == null) {

            return false;

        }

        ListNode slow = head;

        ListNode fast = head;

        while (fast != null && fast.next != null) { 

            slow = slow.next;

            fast = fast.next.next;

            if (fast == slow) {

                return true;

            } 

        }

        return false;

    }

}