Practice: Valid Parentheses

Problem: https://leetcode.com/problems/valid-parentheses/

Recognition reminder: every closing bracket must match the most recently opened unmatched bracket. “Most recent first” is last in, first out, the plainest stack cue there is.

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: ____

One pass with a stack should do it. Know what each closing bracket has to check against the top of the stack, and what an empty stack at the end (and an empty stack mid-way) tells you.

Where your code goes

Write your solution as a function is_valid(s: str) -> bool in a file named solution.py in your own work repo (see getting-started.md), not in the course repo. 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?