Given the `head` of a linked list, return the node where the cycle begins. If there is no cycle, return `null`.

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 (0-indexed). It is `-1` if there is no cycle. Note that `pos` is not passed as a parameter.

Do not modify the linked list.

Example 1:

    Input: head = [3,2,0,-4], pos = 1
    Output: tail connects to node index 1
    Explanation: There is a cycle in the linked list, where tail connects to the second node.

Example 2:

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

Example 3:

    Input: head = [1], pos = -1
    Output: no cycle
    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?