题目：

Given a linked list, determine if it has a cycle in it.

Follow up:

Can you solve it without using extra space?

让head不断走一步，如果cur不为null，走两步，这样如果存在cycle，cur会追上head。

/** * 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) {        ListNode* cur = head;                while (cur && cur->next != head){            cur = cur->next;            if (cur)                cur = cur->next;            else                break;            head = head->next;        }        if (!cur) return false;        return true;    }};