题目描述：
Given a linked list, determine if it has a cycle in it.
Follow up:
Can you solve it without using extra space?
解题方案：
使用快慢指针，如果有环，快指针肯定会追上慢指针。下面是该题的代码：
1 class Solution { 2 public: 3 bool hasCycle(ListNode *head) { 4 struct ListNode *slow = head; 5 struct ListNode *fast = head; 6 7 while (fast != NULL && fast->next != NULL) { 8 slow = slow->next; 9 fast = fast->next->next; 10 11 if (slow == fast) { 12 return true; 13 } 14 } 15 return false; 16 } 17 };
原文：http://www.cnblogs.com/skycore/p/4025379.html