Given a linked list, determine if it has a cycle in it.
Follow up:
Can you solve it without using extra space?
这道题是快慢指针的经典应用。只需要设两个指针，一个每次走一步的慢指针和一个每次走两步的快指针，如果链表里有环的话，两个指针最终肯定会相遇。实在是太巧妙了，要是我肯定想不出来。代码如下：
C++ 解法：
class Solution { public: bool hasCycle(ListNode *head) { ListNode *slow = head, *fast = head; while (fast && fast->next) { slow = slow->next; fast = fast->next->next; if (slow == fast) return true; } return false; } };
Java 解法：
public class Solution { public boolean hasCycle(ListNode head) { ListNode slow = head, fast = head; while (fast != null && fast.next != null) { slow = slow.next; fast = fast.next.next; if (slow == fast) return true; } return false; } }
本文转自博客园Grandyang的博客，原文链接：单链表中的环[LeetCode] Linked List Cycle ，如需转载请自行联系原博主。
