Hard data structure and algorithm practice puzzle.
Given a list of numbers, arrange them so that joining them together makes the biggest possible number, and return it as a string. Sorting by value doesn't work, since 3 should go before 30. Instead, compare two numbers a and b by trying both orders: put a first if a + b reads bigger than b + a. If the result starts with 0, it's all zeros, so return "0".
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 Packet Calendar Merger, a pipeline you can run and download. See every Software Engineer project.
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