Easy data structure and algorithm practice puzzle.
Given a binary search tree, return its smallest value, or null if it's empty. Keep going left until there's no left child. In a search tree, the smallest value is always all the way down the left side, so you never look right.
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 Frame Search Index, a pipeline you can run and download. See every Software Engineer project.
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