Linked List Cycle II

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?

 

public class Solution {
    public ListNode detectCycle(ListNode head) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        ListNode fast = head, slow = head;
        while(fast != null && fast.next != null){
            fast = fast.next.next;
            slow = slow.next;
            if(fast == slow){
                slow = head;
                while(fast != slow){
                    fast = fast.next;
                    slow = slow.next;
                }
                return slow;
            }
        }
        return null;
    }
}