Practice: Longest Common Subsequence

Problem: https://leetcode.com/problems/longest-common-subsequence/

Recognition reminder: you are comparing two strings position by position to find the longest subsequence common to both (a subsequence keeps order but need not be contiguous). Two positions, one in each string, name the subproblem, and a longer prefix pair builds from shorter ones depending on whether the current characters match. That is 2-D string DP, the direct sibling of the canonical Edit Distance.

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: ____

The match case and the mismatch case read different neighbors. Be able to say which cells each reads and why, and whether you need the whole table or just two rows.

Where your code goes

Write your solution in your own work repo (see getting-started.md), not in this folder. 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?