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

Follow up:

Can you solve it without using extra space?

思路

两个指针。一个每次走一步，另一个每次走两步，如果有环，会相遇。

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

做题是两个细节：

1. 上来判断head是否为空

2. 循环的条件是(pre != NULL && pre->next != NULL)