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 }