Easy data structure and algorithm practice puzzle.
Given the cache's keys in order of use, oldest first, and a key that was just used, return the new order. Move that key to the newest end. If it wasn't there, just add it at the end. This one step is what turns a cache that throws away the oldest entry into one that throws away the least recently used.
Solve it three ways: snap the jigsaw pieces together, fill in the blanks, or type it from scratch against the tests. This one is free.
Every line of the solution comes with a note explaining why it is there, and a step by step replay shows the code running. At level 3, where you type it yourself, you can ask for an AI mock interview on this problem that ends with a hiring manager's scorecard.
Then grow it into a real project with the Software Engineer button: this puzzle becomes the Trip LRU Service, a pipeline you can run and download. See every Software Engineer project.
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