Easy data structure and algorithm practice puzzle.
Given an array, return a new array where each spot holds the total of everything up to and including it. So [1, 2, 3, 4] becomes [1, 3, 6, 10]. Keep a running total and push it after each number. This is a PREFIX SUM, and once you have one, the total of any stretch is a single subtraction.
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 Shot Growth Model, a pipeline you can run and download. See every Software Engineer project.
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