Linked List Cycle

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?
采用快慢指针实现，快指针总会赶上慢指针。

/**
* 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(NULL == head||NULL == head->next) return false;
ListNode *quick = head,*slow =head;
while(quick!= NULL && quick->next != NULL)
{
quick = quick->next->next;
slow = slow->next;
if(quick == slow)
return true;
}
return false;
}
};