Algorithm :- Detect Cycle in a graph

Question

Write $O(n)$ time algorithm to find any cycles in a graph. Print NONE otherwise

Comments

Graph is directed or not ?

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Can we have general for both directed and undirected?

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For LL: https://www.geeksforgeeks.org/detect-loop-in-a-linked-list/

For Graph:

Floyd’s Cycle-Finding Algorithm:
This is the fastest method. Traverse linked list using two pointers.  Move one pointer by one and other pointer by two.  If these pointers meet at some node then there is a loop.  If pointers do not meet then linked list doesn’t have loop.

Ref: geeksforgeeks