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?

/**
 * 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)return null;
ListNode slow=head;
ListNode fast=head;

do{
if(fast==null||fast.next==null)return null;
fast=fast.next.next;
slow=slow.next;
} while(fast!=slow);
       
       slow=head;
while(slow.val!=Integer.MAX_VALUE){
   //這樣能夠經過。。。投機？
   slow.val=Integer.MAX_VALUE;
slow=slow.next;

}
return slow;
}
}