Two Pointers on linked list
Last updated: August 13, 2025
Quick Overview
Given a singly linked list, implement a function that uses the two-pointer technique to determine if there is a cycle in the list. The function should return true if a cycle exists and false otherwise. You may not use any extra space for storing nodes.
Zillow
August 13, 20254
6
884 solved
Given a singly linked list, implement a function that uses the two-pointer technique to determine if there is a cycle in the list. The function should return true if a cycle exists and false otherwise. You may not use any extra space for storing nodes.
This coding problem is frequently asked during Take-home Project at Zillow. The interviewer is testing your ability to translate a problem into clean, working code while discussing time and space complexity. Zillow expects candidates to write production-quality code, not just solve the puzzle.
What the Interviewer Expects
- Recognize the underlying problem pattern (sliding window, two pointers, BFS/DFS, etc.)
- Discuss multiple approaches and trade-offs before coding
- Implement an optimal solution with clean, production-quality code
- Handle all edge cases including boundary conditions and invalid input
- Optimize both time and space complexity with clear justification
- Test your solution systematically with well-chosen examples
Key Topics to Cover
How to Approach This
- Clarify input constraints and edge cases before writing code.
- Walk through your approach verbally and confirm with the interviewer before coding.
- Start with a brute force solution, then optimize. Mention time and space complexity.
- Test your solution with examples, including edge cases like empty input or duplicates.
- Consider common patterns: sliding window, two pointers, hash map, BFS/DFS, dynamic programming.
Possible Follow-up Questions
- What if the input doesn't fit in memory?
- Can you optimize the space complexity of your solution?
- Can you solve this in a single pass?
- Can you solve this iteratively instead of recursively (or vice versa)?
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Problem Analysis
The problem at hand is to determine if a singly linked list has a cycle. This is a classic scenario where the two-pointer technique, also known as Floyd's Tortoise and Hare algorithm, is applicable. T...
Approach
- Initialize two pointers, 'slow' and 'fast'. Set both to the head of the linked list.
- Traverse the linked list using a loop. In each iteration, move 'slow' one step forward and 'fast' two steps...
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