Detect a Linked List Loop | Dataford Interview Questions

An Equifax security-review workflow is represented as a singly linked chain. Detect whether following the next reference from the starting node eventually revisits a node.

Implement has_cycle(next_indices, head). Use Floyd's tortoise-and-hare technique and require constant extra space.

Formal Specification

next_indices is an array where index i represents a node.
next_indices[i] is the index of the next node, or -1 if node i terminates the chain.
head is the starting node index, or -1 for an empty chain.
Return True if a cycle is reachable from head; otherwise return False.

Constraints

0 <= len(next_indices) <= 100,000
Each next_indices[i] is -1 or an index from 0 to len(next_indices) - 1
head is -1 or an index from 0 to len(next_indices) - 1
Use O(1) auxiliary space

Function Signature

def has_cycle(next_indices, head):

Your question is Detect a Linked List Loop. Start with the requirements in the Question tab.

Run and submit as often as you like. When you're ready, talk me through your approach or go straight to the code.

Problem