Detect Cycle in Linked List | Dataford Interview Questions

At Slack, a service may receive malformed linked-list structures from an in-memory cache. Write a function that determines whether a singly linked list contains a cycle.

A cycle exists if following next pointers eventually revisits a previously seen node instead of reaching None.

Formal Specification

Implement a function that takes the head of a singly linked list and returns a boolean:

Input: head, the first node of a singly linked list, or None
Output: True if the list contains a cycle, otherwise False

Each node has:

val: integer value
next: reference to the next node or None

Constraints

0 <= number of nodes <= 10^5
-10^9 <= node.val <= 10^9
Each node has a next pointer to another node or None
The input may contain a cycle
Prefer O(1) extra space

Function Signature

def has_cycle(head):

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