Easy data structure and algorithm practice puzzle.
Given a binary tree, return how many nodes it has. It's the same recursion as summing: count 1 for yourself, plus whatever each subtree reports, with an empty branch reporting 0. Some trees store this count at every node so they can find the kth smallest value fast.
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 Clause Taxonomy Builder, a pipeline you can run and download. See every Software Engineer project.
Browse all 576 puzzles