Medium data structure and algorithm practice puzzle.
Checking every pair of objects for collisions is O(n squared). A broad phase skips most of them: drop each object into a spatial grid cell and only compare objects that share a cell. Given points [x, y] and a cell size, return how many pairs of points share a cell. Use a hash map from cell to count. Each new point pairs with everyone already in its cell, so add the current count before increasing it. Negative positions round down into negative cells.
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 Entity Session Roll-up, a pipeline you can run and download. See every Software Engineer project.
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