Linked List Cycle

Easy · Linked List

You are given the head of a singly linked list. Some implementations of such a list can be malformed so that a node's next pointer loops back to an earlier node instead of eventually reaching null. Determine whether the given list contains such a loop anywhere in it. Return true if a cycle exists, and false if the list terminates normally.

Examples

Input:  head = [3,2,0,-4], the last node points back to index 1
Output: true
Why:    Following next pointers from the last node leads back into the list, so it never reaches null and forms a loop.

Input:  head = [1,2], the last node points back to index 0
Output: true
Why:    The tail connects to the head, so traversal repeats forever.

Input:  head = [1]
Output: false
Why:    The single node's next pointer is null, so traversal ends without revisiting anything.

Constraints

0 <= number of nodes <= 10^4, -10^5 <= node value <= 10^5, the list may loop back to an earlier node in it

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Practise Linked List Cycle

This statement is written for CodeSpeek. The problem is part of the NeetCode 150 list; Watch NeetCode's explanation of Linked List Cycle. Reference solutions from the NeetCode repository (MIT) are used to verify our tests.