Medium data structure and algorithm practice puzzle.
Given what an investment cost and a list of monthly cash returns, return the first month (counting from 1) where the running total covers the cost. If it never does, return -1. Keep a running total and check it after every month. Months can be negative, so check every month, not just the end.
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 Sampler, a pipeline you can run and download. See every Software Engineer project.
Browse all 576 puzzles