linked-list-cycle-ii Java code

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) {      ListNode slow=head;      ListNode fast=head;      while(fast!=null&&fast.next!=null){          fast=fast.next.next;          slow=slow.next;          if(fast==slow){             ListNode slow2=head;              while(slow2!=slow){               slow=slow.next;                slow2=slow2.next;               }              return slow;          }      }         return null;    }}