leetcode--Linked List Cycle II

Given a linked list, return the node where the cycle begins. If there is no cycle, returnnull.

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 ListNode detectCycle(ListNode head) {        if(null == head || null == head.next) return null;        ListNode slow = head;        ListNode fast = head;        while(slow != null && fast != null && fast.next != null){            slow = slow.next;            fast = fast.next.next;            if(slow == fast){                slow = head;                while(slow != fast){                    slow = slow.next;                    fast = fast.next;                }                return slow;            }        }        return null;    }}