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) {

//     }
// }
public class Solution {
    public boolean hasCycle(ListNode head) {
    if (head == null || head.next == null ) {return false;}
    ListNode slow = head;
    ListNode fast = head;
    while (fast != null && fast.next != null ) {
        slow = slow.next;
        fast = fast.next.next;
        if (slow == fast) {
            return true;
        }
    }
    return false;
     }
}