linked-list-cycle——链表、判断是否循环链表、快慢指针

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

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

若在while开始时判断fast==slow，会出现误判，即第一次循环时fast必定等于slow

 /**
  * 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 *slow=head,*fast=head;
         while(fast!=NULL&&fast->next!=NULL){

             fast=fast->next->next;
             slow=slow->next;
             if(fast==slow)
                 return true;

         }
         return false;
     }
 };