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) {
       /* if(head==null) return false;
        ListNode fast=head;
        ListNode slow=head;
        while(fast!=null){
            
            slow=slow.next;
            if(fast.next!=null){
              fast=fast.next.next;  
            }else {
                return false;
            }
            
           if(slow==fast) return true;
        }
        return false;*/
        
        ListNode fast=head;
        ListNode slow=head;
        while(fast!=null&&fast.next!=null){
            slow=slow.next;
            fast=fast.next.next;
            if(slow==fast)break;
        }
        if(fast==null||fast.next==null){
            return false;
        }
        return true;
        
    }
}