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

Follow up:
Can you solve it without using extra space?

这个题目用快慢指针来做，重点在于代码怎么实现的简洁方便理解。

这里用快指针来判断链表是不是有NULL，没有NULL那再继续走，看是否能与慢指针遇上

class Solution {
public:
    bool 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;
    }
};

leetcode--Linked List Cycle--判断链表是否有环