Linked List Cycle

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

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

题目的意思是判断链表中有没有环

思路：

定义两个指针，一个慢指针，一个快指针，慢指针一次走两步，快指针一次走一步，如果有环，那么慢、快指针一定会在环中相遇

代码如下:

/** * 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)            return false;        ListNode *result=new ListNode(0);        result->next=head;        ListNode *p,*q;        p=result;        q=result;                while(q->next!=NULL&&q->next->next!=NULL)        {            q=q->next->next;            p=p->next;            if(q==p)            {                return true;            }                   }                return false;    }};