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?

一次过，还是Runner Technique的方法

 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         if (head == null || head.next == null) return null;
15         ListNode runner = head;
16         ListNode walker = head;
17         while (runner != null && runner.next != null) {
18             runner = runner.next.next;
19             walker = walker.next;
20             if (runner == walker) break;
21         }
22         if (runner != walker) return null;
23         runner = head;
24         while (runner != walker) {
25             runner = runner.next;
26             walker = walker.next;
27         }
28         return walker;
29     }
30 }