Solved is not the same as remembered. You can solve an exercise, understand it completely, close the tab, and fail to reproduce it three days later. That is not a failure of understanding—it is how memory works. This site is built around that fact. The exercises teach, and the review schedule makes what you learned stick. When a “Due” badge appears next to an exercise you already solved, it is not nagging. The schedule is telling you that today is the day your memory needs the exercise again.
Forgetting is the default
Everything you learn decays. The curve is steep: a day after learning something new, most of it is gone unless something pulls it back. Reading the explanation again does not pull it back. It feels productive, but recognition is not recall. With the explanation in front of you, you recognize the ideas and mistake that ease for knowledge. Close the book. The knowledge goes with it.
The remedy is retrieval, not more reading: close the book and pull the answer out of memory. Remembering strengthens the memory. Each successful recall makes the trace stronger and the next recall easier. That is why the site’s reviews reopen an exercise with the answer hidden and ask you to do it again. Its memory cards ask you to recall a snippet from a blank page rather than pick it from a list. The study view says it outright: studying never counts as solved. The solve happens when you can do it without the reference.
Spacing: harder now, remembered longer
If retrieval practice is the engine, spacing is the fuel. Cramming—all the practice in one sitting—feels intense and effective, then evaporates. The same amount of practice spread across time sticks far longer. The reason is the forgetting curve again. Each spaced session meets the memory just as it begins to decay and pulls it back. Rescuing a fading memory does more work than reinforcing a fresh one.
This is why the site does not let you stay on one exercise until you are bored of it. When you solve something, it comes back after a short gap—a day, say. Solve it again, and the gap grows. Success stretches the interval; failure tightens it. The exercise comes back right before you would have forgotten it. That is when a review does the most good. The reviews get harder in one sense: the gaps grow, so each recall has to work harder. That is exactly the point. A review that feels easy was scheduled too late or too early. A review that takes real effort is doing the work.
Interleaving: mixing beats drilling
Practising one skill until it is perfect feels like mastery and produces narrow expertise. Interleaving—mixing related skills in the same session—feels messier and produces transferable skill. Every switch forces you to decide again which technique this problem needs. The site’s review queue mixes the exercise families on purpose: a due substitution sits next to a due logic exercise, and the “Up next” recommendation points at what is most due across the whole map, not at another instance of what you just did. Each different problem shape forces the “which technique?” decision that tests rarely require and practice should.
The two clocks
The review schedule comes in two flavours. Exercises run on a binary clock: solved or not, with a comfort counter that stretches the gap after repeated successes and shrinks it after a stumble. Memory cards run on a graded clock: each recall is scored from 0 to 5, and the score drives the next interval in the classic spaced-repetition way—a confident 5 stretches the gap a lot, while a shaky 2 brings it back close. Both clocks have the same shape: the gap grows with success, collapses on failure, and the “Due” badge is the clock telling you the moment has arrived.
What failure on a review means
Failing a review is not a setback. The site does not treat it like one. It is information: the interval was too long for that item, so the clock shortens it and the next review comes sooner. A failed review was still a review. You attempted retrieval, got the answer wrong, and punched the memory back into focus more effectively than a reread would have. The exercises that come back “Due” with a wrong answer are not punishments. They are the system re-teaching at the right distance.
Why this matters here
The exercise families are the curriculum, and the schedule is the teacher’s assistant. The site would not be honest about learning if it shipped one without the other. The memory cards and the recall exercises exist for one reason: to make retrieval a first-class activity—not a side effect of practice, but the point of the review. Understanding the mechanism changes what the “Due” badge feels like. It is not homework accumulating. It is the system holding the door open at the exact moment your memory needs to walk through it again.
Where to go next
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How a strong solver approaches a problem
- what the review is for: the “which technique?” decision, practiced until it is a reflex.
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Algorithms past the interview
- why the mix of exercise families is the point: retrieval on unfamiliar ground is the skill the interviews (and the job) actually reward.