Easy data structure and algorithm practice puzzle.
Given a number, return the sum of its digits, using recursion. A one-digit number is its own sum. Otherwise, it's the last digit (n mod 10) plus the digit sum of everything before it (n divided by 10, rounded down). The base case is what stops the recursion.
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 Ordered Index, a pipeline you can run and download. See every Software Engineer project.
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