Hard data structure and algorithm practice puzzle.
Given an array, return every unique set of three numbers that adds up to 0. Sort it. Then fix one number and use two pointers on the rest: if the sum is too small, move left forward, and if it's too big, move right back. Skip any value that's the same as the one before it, so you don't repeat answers. That's O(n squared).
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 Read Throttle, a pipeline you can run and download. See every Software Engineer project.
Browse all 576 puzzles