題目：

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?

鏈接： http://leetcode.com/problems/linked-list-cycle-ii/

6/11/2017

註意

第11，12行沒有放在第7行是為了排除第一次最開始slow, fast都在head的情況，但是也不應該在第7行判斷是否2者當時是head，有可能環的起點就是head

 1 public class Solution {
 2     public ListNode detectCycle(ListNode head) {
 3         if (head == null) {
 4             return head;
 5         }
 6         ListNode slow = head, fast = head;
 7         while (fast != null && fast.next != null) {
 8             slow = slow.next;
 9             fast = fast.next;
 
10             fast = fast.next;
11             if (slow == fast) {
12                 break;
13             }
14         }
15         if (fast == null || fast.next == null) {
16             return null;
17         }
18         fast = head;
19         while (fast != slow) {
20             fast = fast.next;
 
21             slow = slow.next;
22         }
23         return fast;
24     }
25 }

這道題應該是記下來的，但是分析不會。

更多討論

https://discuss.leetcode.com/category/150/linked-list-cycle-ii

142. Linked List Cycle II