Detect a loop in a linked list
💡 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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