How to Solve Detect Cycle in Linked List on LeetCode
Detect Cycle in Linked List is a classic interview question focusing on pointers and linked list manipulation.
Problem Statement
Given the head of a linked list, determine if the linked list has a cycle. Return true if there is a cycle in the linked list, otherwise return false.
Example:
Input: head = [3,2,0,-4], pos = 1
Output: true
Explanation: The tail connects to the node at position 1 (cycle exists).
Why This Problem Matters for Interviews
- Essential for mastering linked list problems
- Introduces fast/slow pointers and cycle detection
- Forms basis for advanced linked list algorithms
Approach to Detect Cycle in Linked List
Fast and Slow Pointers (Floyd's Algorithm)
Move two pointers at different speeds. If they ever meet, a cycle exists.
Time Complexity: O(n)
Space Complexity: O(1)
def hasCycle(head):
slow = fast = head
while fast and fast.next:
slow = slow.next
fast = fast.next.next
if slow == fast:
return True
return False
Key Interview Talking Points
- Set-based approach vs pointer-based approach
- How the algorithm guarantees cycle detection
- Edge cases: empty list, single node
Big O Complexity Recap
| Approach | Time Complexity | Space Complexity |
|---|---|---|
| Fast/Slow Pointers | O(n) | O(1) |
Pro Interview Tips
- Draw pointer positions at each iteration
- Explain why O(1) space is optimal
- Follow-ups: find cycle entry point