这个题目的只要设置一个快步走的指针和一个慢步走的指针即可，如果有环，两者一定会重逢
Given a linked list, determine if it has a cycle in it.
Follow up:
Can you solve it without using extra space?
/** * Definition for singly-linked list. * struct ListNode { * int val; * ListNode *next; * ListNode(int x) : val(x), next(NULL) {} * }; */ class Solution { public: bool hasCycle(ListNode *head) { if (head == NULL || head->next == NULL||head->next->next==NULL)return false; ListNode *p = head->next,*s=head->next->next; while (s&&s->next) { if (p == s)return true; p = p->next; s = s->next->next; } return false; } };
LeetCode 141. Linked List Cycle
原文：http://www.cnblogs.com/csudanli/p/5747325.html