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;
    }
}