LeetCode 141: 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?

/**
 * 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) {
        // Define two pointer: Fast and Slow
        // If they meet, then there is cycle in the linked list
        ListNode fast = head, slow = head;
        
        while (fast != null && fast.next != null)
        {
            fast = fast.next.next;
            slow = slow.next;
            
            if (fast == slow)
                return true;
        }
        
        return false;
    }
}