 ## Challenge Statement

• Given `head`, the head of a linked list, determine if the linked list has a cycle in it.
• There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the `next` pointer. Internally, `pos` is used to denote the index of the node that tail’s `next` pointer is connected to. Note that `pos` is not passed as a parameter.
• Return `true` if there is a cycle in the linked list. Otherwise, return `false`.
• This challenge corresponds to LeetCode #141.

### 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.

### Example 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).``````

### Example 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.``````

### Example 3:

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

## Solution

Below is my solution and some test cases. This solution has a linear time complexity O(n) and a constant space complexity O(1), where n is the length of the linked list.