Easy data structure and algorithm practice puzzle.
Given n, return n factorial: 1 x 2 x 3 x ... x n. By definition, 0 factorial is 1. Start at 1 and multiply your way up. It counts the ways to arrange n different items, and it grows ridiculously fast. 20 factorial already overflows a 64-bit integer.
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 Reading Ledger Balancer, a pipeline you can run and download. See every Software Engineer project.
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