Linked List Cycle

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`.   **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 = [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.   **Follow up:** Can you solve it using `O(1)` (i.e. constant) memory?