142. Linked List Cycle II
Similiar problem:141. Linked List Cycle
Given a linked list, return the node where the cycle begins. If there is no cycle, return null.
Note: Do not modify the linked list.
Follow up:
Can you solve it without using extra space?
真的不懂..
参考：http://www.cnblogs.com/abc-begin/p/7666817.html
https://blog.csdn.net/willduan1/article/details/50938210
先看160题吧！
160. Intersection of Two Linked Lists
题外话：如果想计算环的大小，怎么算？解：第一步，判断是否有环，即让快慢指针相遇，记为c点，从c点开始，让慢指针行走（此时快指针固定在c点，等候慢指针），直到快慢指针相遇，此时慢指针行走的步数就是环的大小。