leetcode 141 Linked List Cycle Hash fast and slow pointer

Problem describe:https://leetcode.com/problems/linked-list-cycle/

Given a linked list, determine if it has a cycle in it.
 
To represent a cycle in the given linked list, we use an integer pos which represents the position (-indexed) in the linked list where tail connects to. If pos is -, then there is no cycle in the linked list.
 
Example :
 
Input: head = [,,,-], pos =
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the second node.
 
Example :
 
Input: head = [,], pos =
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the first node.
 
Example :
 
Input: head = [], pos = -
Output: false
Explanation: There is no cycle in the linked list.
 
Follow up:
 
Can you solve it using O() (i.e. constant) memory?
 

Ac Code: (Hash)

 /**
  * 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) {
         unordered_set<ListNode*> visit;
         while(head)
         {
             if(visit.count(head)!=) return true;
             visit.insert(head);
             head = head->next;
         }
         return false;
     }
 };

Fast and Slow Pointer

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