public class Solution
extends Object
141 - Linked List Cycle\.
Easy
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?