Medium data structure and algorithm practice puzzle.
Given a list of wall heights, pick two walls that hold the most water between them and return that amount. Water is the shorter wall times the distance between them. Start with one pointer at each end and always move the SHORTER wall inward. The short wall limits the water, so keeping it while getting narrower can only do worse.
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 Clause Matcher, a pipeline you can run and download. See every Software Engineer project.
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