Hard data structure and algorithm practice puzzle.
Given n, return the biggest number no larger than n whose digits never go down from left to right. So 332 becomes 299. Walk the digits from right to left. Whenever a digit is bigger than the one after it, lower it by 1 and remember that spot. At the end, turn every digit from the leftmost spot you marked onward into a 9.
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 Login Budget Allocator, a pipeline you can run and download. See every Software Engineer project.
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