Detect if a Linked List is Circular in DSA C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does it mean for a linked list to be circular?

A linked list is circular if its last node points back to any previous node in the list, creating a loop instead of ending with a null pointer.

intermediate

Which pointer technique is commonly used to detect a cycle in a linked list?

The Floyd’s Cycle-Finding Algorithm uses two pointers: a slow pointer that moves one step at a time and a fast pointer that moves two steps at a time. If they meet, the list is circular.

intermediate

What is the time complexity of detecting a circular linked list using Floyd’s Cycle-Finding Algorithm?

The time complexity is O(n), where n is the number of nodes in the linked list, because each pointer traverses the list at most once.

beginner

Why can’t we just check if the last node’s next pointer is NULL to detect circularity?

Because in a circular linked list, the last node’s next pointer points to a previous node, not NULL. But if the list is very large or has no explicit end, we need a method to detect loops without traversing infinitely.

beginner

What happens if the fast pointer reaches NULL in Floyd’s Cycle-Finding Algorithm?

If the fast pointer reaches NULL, it means the linked list has an end and is not circular.

What does the slow pointer do in Floyd’s Cycle-Finding Algorithm?

A Moves two nodes at a time

B Jumps to the end of the list

C Moves one node at a time

D Stays at the head node

The slow pointer moves one node at a time to help detect cycles by eventually meeting the fast pointer if a loop exists.

If a linked list is not circular, what will happen to the fast pointer?

A It will eventually point to NULL

B It will meet the slow pointer

C It will loop infinitely

D It will point to the head node

In a non-circular linked list, the fast pointer will reach the end and point to NULL.

Which of these is a sign that a linked list is circular?

A Last node’s next pointer is NULL

B Slow and fast pointers meet at some node

C List length is zero

D Head node is NULL

If slow and fast pointers meet, it means the list has a cycle and is circular.

What is the main advantage of Floyd’s Cycle-Finding Algorithm?

A Uses extra memory to store visited nodes

B Sorts the linked list

C Deletes circular nodes

D Detects cycle without extra memory

Floyd’s algorithm detects cycles using two pointers without extra memory.

What is the first step in detecting if a linked list is circular?

A Initialize slow and fast pointers at head

B Check if head is NULL

C Count the number of nodes

D Reverse the linked list

The algorithm starts by setting both slow and fast pointers at the head node.

Explain how Floyd’s Cycle-Finding Algorithm detects a circular linked list.

Think about how two runners on a track can meet if the track loops.

Describe why detecting a circular linked list is important before traversing it.

Imagine walking in circles without knowing it.