Hard data structure and algorithm practice puzzle.
Given a grid where 0 is a wall and any other number is what it costs to step onto that tile (grass 1, swamp 5), return the cheapest cost from the top-left corner to the bottom-right, moving up, down, left or right, or -1 if there's no way through. Use A*: always expand the open tile with the smallest cost so far plus a heuristic guess at what's left. The guess here is the Manhattan distance to the goal. It's admissible, meaning it never overestimates, because every tile costs at least 1.
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 Entity Route Planner, a pipeline you can run and download. See every Software Engineer project.
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