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:**

![](https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist.png)

**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:**

![](https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist_test2.png)

**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:**

![](https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist_test3.png)

**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 <code>[0, 10<sup>4</sup>]</code>.
*   <code>-10<sup>5</sup> <= Node.val <= 10<sup>5</sup></code>
*   `pos` is `-1` or a **valid index** in the linked-list.

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