Hard data structure and algorithm practice puzzle.
Given a binary tree, return true if it's a valid binary search tree, where everything on the left is smaller and everything on the right is bigger. Just checking each node against its children isn't enough. A value can be fine with its parent and still be on the wrong side of its grandparent. Pass down the allowed range and tighten it at every step.
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 Document Taxonomy Builder, a pipeline you can run and download. See every Software Engineer project.
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