Detect a loop in a linked list

Q: Which algorithm detects loops in linked lists?

Floyd's Cycle Detection algorithm.

💡 Concept: Loop Detection in Linked List

Detecting if a linked list contains a loop (cycle) to prevent infinite traversal.

📘 Quick Intro

Loops in linked lists cause infinite loops during traversal; detecting them is essential for robustness.

🧠 Analogy

Like detecting if a racetrack loops back onto itself, causing you to run endlessly without a finish line.

🔧 Technical Explanation

Use Floyd’s Cycle Detection (Tortoise and Hare) algorithm.
Two pointers move at different speeds through the list.
If pointers meet, a loop exists.
If a pointer reaches null, no loop exists.
Algorithm runs in O(n) time and O(1) space.

🎯 Use Cases

✅ Detecting corrupted linked list data structures.
✅ Debugging infinite loops in algorithms.
✅ Ensuring safety in data processing pipelines.
✅ Interview algorithm questions.

💻 Code Example


// Detect loop using Floyd's cycle-finding algorithm
public bool HasLoop(Node head) {
    if (head == null) return false;

    Node slow = head;
    Node fast = head;

    while (fast != null && fast.Next != null) {
        slow = slow.Next;
        fast = fast.Next.Next;

        if (slow == fast) {
            return true; // Loop detected
        }
    }
    return false; // No loop
}

❓ Interview Q&A

Q1: What is the purpose of detecting loops in linked lists?
A: To prevent infinite traversal and program crashes.

Q2: Which algorithm is commonly used?
A: Floyd's Cycle Detection (Tortoise and Hare).

Q3: What is the time complexity?
A: O(n).

Q4: What is the space complexity?
A: O(1).

Q5: Can you detect loops without extra memory?
A: Yes, using two-pointer approach.

Q6: What happens if list is empty?
A: No loop exists.

Q7: Can multiple loops exist?
A: A single loop is considered in this context.

Q8: How to remove loops?
A: By breaking the loop reference manually.

Q9: What is the fast pointer speed?
A: Moves two nodes at a time.

Q10: What is the slow pointer speed?
A: Moves one node at a time.

📝 MCQs

Q1. Which algorithm detects loops in linked lists?

Depth First Search
Floyd's Cycle Detection
Breadth First Search
Dijkstra's Algorithm

Q2. What is the time complexity?

O(1)
O(n)
O(n^2)
O(log n)

Q3. What is the space complexity?

O(1)
O(n)
O(n^2)
O(log n)

Q4. What speed does the fast pointer move?

One node
Two nodes
Three nodes
Four nodes

Q5. What speed does the slow pointer move?

One node
Two nodes
Three nodes
Four nodes

Q6. What if the list is empty?

Loop exists
No loop
Error
Null pointer exception

Q7. Can this detect multiple loops?

Yes
No
Maybe
Sometimes

Q8. How to remove detected loop?

Ignore
Break reference
Rebuild list
Delete list

Q9. Why detect loops?

Improve speed
Prevent infinite loops
Memory optimization
Better traversal

Q10. Is extra memory needed?

Yes
No
Sometimes
Always

💡 Bonus Insight

Floyd’s cycle-finding algorithm is an elegant solution that uses two pointers and constant space to detect loops efficiently.

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