Hard data structure and algorithm practice puzzle.
Given each month's profit or loss, return the first month where the RUNNING TOTAL goes above 0, counting months from 1. If it never does, return -1. Keep a running total and check it after every month. A company can make money every month and still not break even for a while, because the early losses have to be paid back first.
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 Trade Range Service, a pipeline you can run and download. See every Software Engineer project.
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