Medium data structure and algorithm practice puzzle.
Given your rating and a list of opponents, each with a rating and a deviation, return how much the batch told the ladder about you, as an estimated variance rounded to four decimal places. Close matches teach a lot. Lopsided ones teach almost nothing. Smaller means more informative, so two even games give half the variance of one.
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 Match Stats Roll-up, a pipeline you can run and download. See every Software Engineer project.
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