Day 21

Que: https://leetcode.com/problems/linked-list-cycle/

Case 1: Input: head = [3,2,0,-4], pos = 1
Output: true
Explanation: There is a cycle in the linked list, where the tail connects to the 1st node (0-indexed).

Case 2: Input: head = [1,2], pos = 0
Output: true
Explanation: There is a cycle in the linked list, where the tail connects to the 0th node.

Case 3:
Input: head = [1], pos = -1
Output: false
Explanation: There is no cycle in the linked list.

Constraints:

The number of the nodes in the list is in the range [0, 104].
-105 <= Node.val <= 105
pos is -1 or a valid index in the linked-list.


class Solution {
public:
    bool hasCycle(ListNode *head) {
         if (head == NULL || head->next == NULL) {
            return false;
        } else {
            ListNode *slow = head, *fast = head->next;
            
            while (slow != fast) {
                if (fast == NULL || fast->next == NULL) {
                    return false;
                } else {
                    slow = slow->next;
                    fast = fast->next->next;
                }
            }
            return true;
        }
        
    }
};