Easy data structure and algorithm practice puzzle.
Given a number, add up its digits. If the result has more than one digit, add those up too. Keep going until there's one digit left, and return it. So 942 becomes 15, then 6. Fun fact: the answer is just n mod 9, with 9 standing in for 0, which is the old trick clerks used to check their math.
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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