题目：
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?
Linked List Two Pointers
链接： http://leetcode.com/problems/linked-list-cycle-ii/
6/11/2017
注意
第11，12行没有放在第7行是为了排除第一次最开始slow, fast都在head的情况，但是也不应该在第7行判断是否2者当时是head，有可能环的起点就是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 }
这道题应该是记下来的，但是分析不会。
更多讨论
https://discuss.leetcode.com/category/150/linked-list-cycle-ii
原文：http://www.cnblogs.com/panini/p/6992009.html