Hard data structure and algorithm practice puzzle.
Given a grid of letters and a word, return true if you can spell the word by moving up, down, left, or right, without using any cell twice. Start a depth-first search from every cell. Mark each cell as used when you step into it, and UNMARK it when you back out. That undo step is what makes it backtracking.
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 Cluster Finder, a pipeline you can run and download. See every Software Engineer project.
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