Hard data structure and algorithm practice puzzle.
Given a binary tree, return true if at every node, the left and right subtrees' heights differ by at most 1. Work out heights from the bottom up, and check the difference at each node on the way. One walk does both. Staying balanced is what keeps a search tree at O(log n).
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 Event Taxonomy Builder, a pipeline you can run and download. See every Software Engineer project.
Browse all 576 puzzles