Medium data structure and algorithm practice puzzle.
Given a binary tree, return its values in postorder: the left subtree, then the right subtree, then the node. Recurse, recording each value after both children are done. Children always finish before their parent, which is the order you'd delete a tree in.
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 Step Taxonomy Builder, a pipeline you can run and download. See every Software Engineer project.
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