Count cycle in linked list
Last updated: September 13, 2025
Quick Overview
Given a linked list, determine if it contains a cycle and, if so, count the number of nodes in the cycle. Your function should return the count of nodes in the cycle, or 0 if there is no cycle. The input will be the head of the linked list, and the output should be an integer representing the count of nodes in the cycle.
Waymo
September 13, 2025148
3
3,082 solved
Given a linked list, determine if it contains a cycle and, if so, count the number of nodes in the cycle. Your function should return the count of nodes in the cycle, or 0 if there is no cycle. The input will be the head of the linked list, and the output should be an integer representing the count of nodes in the cycle.
This coding problem is frequently asked during Onsite at Waymo. The interviewer is testing your ability to translate a problem into clean, working code while discussing time and space complexity. Waymo expects candidates to write production-quality code, not just solve the puzzle.
What the Interviewer Expects
- Identify the correct data structure and algorithm for the problem
- Write clean, bug-free code with proper variable naming
- Analyze time and space complexity correctly
- Handle basic edge cases (empty input, single element)
- Communicate your thought process while coding
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
- Can you solve this iteratively instead of recursively (or vice versa)?
- What if the input doesn't fit in memory?
- How would you parallelize this solution?
- What happens if the input contains duplicates?
Sharpen Your Skills on Codemia
Practice similar problems with our interactive workspace, get AI feedback, and track your progress.
Practice DSA ProblemsSample Answer
Problem Analysis
To determine if a linked list contains a cycle and to count the number of nodes in the cycle, we can utilize the Floyd's Tortoise and Hare algorithm, which employs the two pointers technique. ...
Approach
- Initialize two pointers,
slowandfast, both pointing to the head of the linked list. - Move
slowone step andfasttwo steps in a loop until eitherfastreaches the end of the list (nul...
Submit Your Answer
