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?

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