Leetcode: Linked List Cycle

Given a linked list, determine if it has a cycle in it.
Follow up:
Can you solve it without using extra space? 

Solution:

Using a fast pointer that advances two nodes each time and a slow pointer that advances on node, we always detect a cycle as the fast pointer cannot overtake the slow one without being in the same node in one of the ``turns".

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public boolean hasCycle(ListNode head) {
        ListNode fast = head;
        ListNode slow = head;
        while(fast!=null && fast.next !=null)
        {
            fast = fast.next.next;
            slow = slow.next;
            if(fast == slow)
                return true;
        }
        return false;
    }
}