leetcode linked list cycle

problem:

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

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

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 Linked List Two Pointers

code:

/**
 * 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,slow=head;
        do{
        if(fast.next==null || fast.next.next==null){
            return null;
        }
        fast=fast.next.next;
        slow=slow.next;
        }while(fast !=slow)
        return true;
    }
}