Easy data structure and algorithm practice puzzle.
Given a binary tree, return the sum of every node's value. Each node reports its own value plus whatever its left and right subtrees report. An empty branch reports 0, and that base case is what keeps the two recursive calls safe. It's the same shape as adding up the size of a folder.
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 Request Ordered Index, a pipeline you can run and download. See every Software Engineer project.
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