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You don't solve the problem in your head, you actually take the time to solve it either on Leetcode or on a piece of paper. I've found that in the long run you don't end up memorizing the problem itself but the technique to solve the problem.

For example, I was learning Manacher's algorithm and I directly put the link to the corresponding Leetcode problem into Anki. Set your minimum interval to 1 day and just follow whatever Anki tells you to study. You don't need many cards to memorize the problem solving techniques, my entire probability deck has under 150 cards and half of that is concepts. It turns out the human brain is pretty good at generalizing from a few problems.

Make sure you don't put every problem you encounter because the review workload will get very heavy very fast if you do that. Only put the challenging ones or the ones you get wrong.