Originally published on LeetCopilot Blog
Solving problems is only half the battle. What you do in the 10–15 minutes after a LeetCode question determines whether you actually improve. Here’s a simple, repeatable review system.
You finish a LeetCode problem, finally get "Accepted," and move on to the next one.
A week later, you see a similar problem—and you’re stuck again.
The issue isn’t that you’re not smart enough. It’s that you’re not reviewing your mistakes in a way that turns them into lasting skills.
This guide shows you a simple, practical system for how to review LeetCode mistakes after solving problems, so each question pushes your skills forward instead of being a one-time event.
TL;DR
- Don’t stop your work at “Accepted”—the real learning happens in the 10–15 minutes after you solve (or fail) a problem.
- A good review system answers: (1) What did I misunderstand? (2) Which pattern was this? (3) Where did my reasoning break? (4) What will I do differently next time?
- You can capture this in a short, structured note for each problem, linking it to a pattern and a common mistake type.
- Beginners often skip post-problem review, hoard long notes they never reread, or only copy the editorial without reflecting on their own thought process.
- With a lightweight template and a DSA learning path, your past mistakes become a map of what to focus on—and tools like AI-guided LeetCode practice can help you revisit them at the right time.
Why Post-Problem Review Matters More Than Problem Count
Solving more problems feels productive. But unreviewed problems fade fast:
- You don’t remember the edge cases you missed.
- You forget why your first approach failed.
- You can’t recall which pattern actually fit the problem.
Reviewing correctly:
- Turns errors into patterns (“I mis-handle off-by-one in sliding window problems”).
- Builds a mental catalog of “when I see X, think of Y.”
- Makes your future practice more targeted and efficient.
A 4-Step Review Framework You Can Use After Every Problem
Step 1: Classify the outcome (2–3 sentences)
Right after finishing:
Outcome:
- Solved by myself? (yes/no)
- Needed hints or editorial? (yes/no)
- Time taken? (rough minutes)
Examples:
- “Solved in ~25 minutes with one small hint.”
- “Got stuck after 40 minutes; needed editorial to see sliding window.”
- “Solved brute force, then optimized with hint.”
This keeps you honest about how much help you needed.
Step 2: Identify the pattern (and pattern confusion)
Ask:
- “What pattern is this really about?” (e.g., sliding window, two pointers, BFS, DP).
- “What wrong pattern did I try first, if any?”
Write:
Pattern:
- Correct: Sliding window over string
- I first tried: Two pointers with sorting (wrong—sorting breaks index relations)
Over time, you’ll see recurring mis-classifications—critical for improving pattern recognition.
Step 3: Analyze the core mistake
Find the main reason you struggled. Common categories:
- Misread constraints
- Picked wrong pattern
- Edge cases (empty input, one element, off-by-one)
- Complexity too high
- Incorrect invariant or window condition
Example:
Mistake:
- I didn’t notice that the substring had to be contiguous, so I thought about subsequences and DP instead of sliding window.
Step 4: Write a “next time, I will…” rule
Turn the mistake into a future guideline:
Next time:
- When I see “contiguous subarray/substring + max/min/longest/shortest”, I will explicitly consider sliding window before DP.
This is the most important line—you’re writing a tiny rule for your future self.
Example: Reviewing a Sliding Window Mistake
Imagine you struggled with “longest substring with at most K distinct characters.”
Your review note might look like:
Problem: Longest substring with at most K distinct characters
Outcome:
- Took ~45 minutes; needed editorial to see the shrinking window condition.
Pattern:
- Sliding window with frequency map
- Initially tried: set-based approach that didn’t shrink correctly
Mistake:
- I didn’t define a clear window invariant. I was adjusting left/right without a rule like “window must have at most K distinct characters.”
Next time:
- For sliding window problems, I will write the invariant explicitly in a comment before coding.
Even a short note like this makes the next similar problem easier.
Simple Code Snippet to Help Surface Mistakes
Sometimes reviewing your code is easier if you can see where it diverges from expectations on small test cases.
A quick helper in Python:
def debug_case(func, args, expected):
result = func(*args)
print("Args: ", args)
print("Expected:", expected)
print("Got: ", result)
if result != expected:
print("→ Mismatch detected. Where does the logic diverge?")
You can call:
debug_case(my_solution, (["ab", "abc"], 3), 2)
Then ask:
- Which branch did I take?
- Did my invariant hold?
- Did I handle the boundary correctly?
This makes your review concrete instead of purely theoretical.
Practical Review Strategies You Can Sustain
Strategy 1: Use a tiny note template per problem
Keep each note short (5–10 lines). A simple template:
Problem:
Outcome:
Pattern:
Mistake:
Next time:
Store these in a single file or tool that supports tags and spaced review, as part of your DSA learning path.
Strategy 2: Tag problems by pattern and mistake type
Example tags:
-
pattern:sliding-window,pattern:two-pointers,pattern:graph-bfs -
mistake:constraints,mistake:edge-cases,mistake:pattern-choice
Later, before interviews, you can quickly review:
- “All sliding window mistakes,” or
- “All constraint-reading mistakes.”
Strategy 3: Schedule micro-review sessions
Once or twice a week, spend 20–30 minutes:
- Skimming 5–10 old notes.
- Re-solving 1–2 key problems from memory.
- Updating notes if your understanding improved.
Tools like an AI-guided LeetCode practice system can help select high-value problems based on your past mistakes.
Common Mistakes When Reviewing (Ironically)
Mistake 1: Writing essays you’ll never reread
Huge paragraphs feel productive but aren’t sustainable.
Fix: Keep notes tiny and focused on the four key questions (outcome, pattern, mistake, next time).
Mistake 2: Only copying editorial code
Copying code into your notes without your own commentary doesn’t help you generalize.
Fix: If you include code, keep it small and annotate why it works or what the key invariant is.
Mistake 3: Never revisiting old notes
Even good notes won’t help if you never review them.
Fix: Tie review sessions to your weekly routine—e.g., Saturday morning = 30 minutes of “mistake review.”
Mistake 4: Ignoring emotional patterns
If certain problem types always frustrate you (e.g., graphs, DP), note that too. It tells you where to intentionally allocate more gentle, structured practice.
FAQ
Q1: How much time should I spend reviewing each problem?
For most problems, 5–10 minutes of focused review is enough. Harder problems or ones that exposed a new pattern might deserve 15–20 minutes and a re-solve in a few days.
Q2: Should I review problems I solved quickly without mistakes?
Yes—but briefly. Confirm the pattern, jot down the key idea, and move on. These help reinforce your strengths.
Q3: What if I have hundreds of solved problems already?
Start by reviewing the most recent 20–30, then gradually work backward focusing on topics you know are weak. You don’t need to retro-review everything.
Q4: How do I know if my review system is working?
You’ll start seeing fewer repeated mistakes of the same type, and new problems will feel more familiar. If you keep making the same mistakes, adjust your “next time” rules and practice them consciously.
Q5: Can I automate any of this?
Yes. For more interview prep guidance, see how to run mock coding interviews by yourself and how to explain your thought process during coding interviews.
Conclusion
Your LeetCode progress is not determined by how many problems you’ve solved—it’s determined by how well you learn from each one.
A lightweight, consistent review routine transforms wrong answers into future strengths:
- Capture outcome, pattern, core mistake, and a “next time” rule.
- Tag and revisit problems by pattern and mistake type.
- Use small, focused review sessions instead of giant, unsustainable ones.
Once you treat review as a first-class part of practice, every problem—right or wrong—moves you measurably closer to being interview-ready.
If you're looking for an AI assistant to help you master LeetCode patterns and prepare for coding interviews, check out LeetCopilot.
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