leetcode Linked List Cycle 链表的快慢指针

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

Follow up:

Can you solve it without using extra space?

/**
* 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;
ListNode* fast = head;
while(fast != NULL) {
fast = fast->next;
if(fast == NULL)
return false;
fast = fast->next;
slow = slow->next;
if(fast == NULL)
return false;
if(slow == fast)
break;
}
return true;
}
};