Linked List Cycle II

Q:

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

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

Solution:

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