Hard data structure and algorithm practice puzzle.
Given n, return how many steps it takes to reach 1. Each step: if n is even, halve it. If it's odd, triple it and add 1. So 6 goes 3, 10, 5, 16, 8, 4, 2, 1, which is 8 steps. Fun fact: nobody has ever proved this reaches 1 for every n. It's been an open problem since 1937.
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 Ledger Balancer, a pipeline you can run and download. See every Software Engineer project.
Browse all 576 puzzles