Problem

In a low-level systems component such as a Meta device runtime, linked structures may become corrupted and form loops. Implement a function that returns whether a singly linked list contains a cycle.

Formal Specification

You are given the head of a singly linked list. Each node has:

• val: an integer value
• next: a reference to the next node or None

Return:

• True if following next pointers eventually revisits a previously seen node
• False otherwise

Your solution should avoid modifying the list.

Examples

Example 1

• Input: head = [3, 2, 0, -4], pos = 1
• Output: True
• Explanation: The tail connects back to the node at index 1, so traversal loops forever.

Example 2

• Input: head = [1, 2], pos = 0
• Output: True
• Explanation: The second node points back to the first node.

Example 3

• Input: head = [1], pos = -1
• Output: False
• Explanation: There is only one node and it does not point to itself.

Constraints

• The number of nodes is in the range [0, 10^4]
• -10^5 <= Node.val <= 10^5
• pos is used only to describe the test setup and is not passed to the function