Leetcode: 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?

 1 /**
 2  * Definition for singly-linked list.
 3  * class ListNode {
 4  *     int val;
 5  *     ListNode next;
 6  *     ListNode(int x) {
 7  *         val = x;
 8  *         next = null;
 9  *     }
10  * }
11  */
12 public class Solution {
13     public ListNode detectCycle(ListNode head) {
14         ListNode current = head;
15         ListNode runner = head;
16         if (head == null || head.next == null || head.next.next == null) return null;
17         current = head.next;
18         runner = head.next.next;
19         while (runner != current && runner.next != null && runner.next.next != null) {
20             runner = runner.next.next;
21             current = current.next;
22         }
23         if (runner == current) {
24             current = head;
25             while (runner != current) {
26                 runner = runner.next;
27                 current = current.next;
28             }
29             return current;
30         }
31         return null;
32     }
33 }