Hard data structure and algorithm practice puzzle.
Given the gas at each station around a loop and the cost to drive to the next one, return the station you can start from to make it all the way around, or -1. If the total gas is less than the total cost, no start works. Otherwise, drive with a running tank. If the tank goes negative at station i, no start up to i can work, so restart from i + 1.
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 Login Calendar Merger, a pipeline you can run and download. See every Software Engineer project.
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