Record the decision that mattered
A useful practice note answers a small set of questions: what approach did you choose, why did it fit the problem, and where did your first attempt go wrong? Start with a sentence about the strategy. Add the rule the algorithm maintains and its time and space costs. Keep the explanation understandable without the final code open beside it.
Avoid turning the note into a transcript of every step you took. Most of those steps will not help when you return to the problem. Save the detail that changes a future decision, such as why a greedy choice fails or why an item must enter the visited set before it is added to a queue.
Keep one concrete counterexample
If an approach or implementation failed, preserve a small input that demonstrates the failure. Include the expected result and the result your code produced. Explain the difference in one or two sentences. This is more actionable than a note that simply says ‘watch out for edge cases.’
For a boundary error, show the boundary. For a mistaken assumption, show an input that does not satisfy it. The example should help you recognize the same mistake in another problem, rather than only reminding you that this particular submission failed.
Distinguish what you knew from what you learned
Record whether you found the key idea yourself, recovered it from a hint, or learned it from the full explanation. That distinction is not a score. It helps you decide whether the next session should focus on recognizing a pattern, implementing it accurately, or explaining why it works.
Make the next step specific. If a hint introduced a useful data structure, try a related problem that uses it. If the idea was clear but the code was fragile, reimplement the core loop and test its boundaries. If you could solve the problem but could not explain it, practice a short explanation before another attempt.
Use the note to guide a later attempt
When you revisit the exercise, begin with the prompt and keep the note closed. Try to reconstruct the strategy and its invariant. If you get stuck, open only the part that addresses the gap, then continue. This gives you a clearer picture of what you can recover without assistance.
Afterward, update the note with any new insight and remove detail you no longer need. Over time, a useful note should become easier to scan. Its job is to guide another attempt and make your reasoning visible, not to preserve every version of the answer.