Linked List Cycle

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

Have you met this question in a real interview? 

Yes

Example

Given -21->10->4->5, tail connects to node index 1, return true

Challenge

Follow up:
Can you solve it without using extra space?

/**

 * Definition for ListNode.

 * public class ListNode {

 *     int val;

 *     ListNode next;

 *     ListNode(int val) {

 *         this.val = val;

 *         this.next = null;

 *     }

 * }

 */ 

public class Solution {

    /**

     * @param head: The first node of linked list.

     * @return: True if it has a cycle, or false

     */

    public boolean hasCycle(ListNode head) {  

        // write your code here

        ListNode slow = head, fast = head;

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

            fast = fast.next.next;

            slow = slow.next;

            if(fast == slow)

                return true;

        }

        return false;

    }

}