Practice: Linked List Cycle

Problem: https://leetcode.com/problems/linked-list-cycle/

Recognition reminder: you are asked whether following .next repeatedly ever loops back on itself. “Does this list have a cycle” is the signal for fast/slow pointers (Floyd’s).

Before you start (the five-beat rhythm)

  1. Your Pattern Card for this week is already written.
  2. Name the pattern aloud and write your approach as a plain-English comment before any code.
  3. Struggle floor: 25 minutes unaided. No hints, no AI, no Discuss tab.
  4. If stuck past the floor, ask the tutor for a hint. Six rungs, one per ask.
  5. Debrief in your commit message before moving to the next problem.

Your target

Fill these in yourself before you look at anyone else’s solution:

Target time complexity:  ____
Target space complexity: ____

A set of visited nodes solves this, but it costs memory. There is a way to answer with two pointers and a constant number of variables. Know why a faster pointer and a slower one must eventually collide if (and only if) there is a loop.

Where your code goes

Write your solution in your own work repo (see getting-started.md), not in this folder. Your solution.py must define the same ListNode class LeetCode gives you (val, next) and a function has_cycle(head) -> bool that returns True if the list contains a cycle and False otherwise. This folder ships only the problem spec and a provided-example test (tests/test_provided.py) so you can check the given cases locally before you submit to LeetCode’s judge. The judge is the oracle; the tutor will not confirm your answer by reading it.

Debrief (paste into your commit message) 

1. What pattern did this turn out to be?
2. What was the trigger phrase or input shape that should have made me reach for it?
3. What was the time and space complexity, and what would dominate at scale?
4. What edge case would have broken my first attempt?
5. What would I do differently in three days when I see this cold?