Hard data structure and algorithm practice puzzle.
Given each person's cost to fly to city A or city B, send exactly half to each and return the smallest total. Sort people by costA minus costB, so the people who save the most by going to A come first. Send the first half to A and the rest to B. Sorting by the difference, not by either cost, is the trick.
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 Matcher, a pipeline you can run and download. See every Software Engineer project.
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