Hard data structure and algorithm practice puzzle.
Given a grid of costs, return the cheapest total to get from the top-left to the bottom-right, moving only right or down. The cheapest way into any cell is its own cost plus the cheaper of the cell above and the cell to its left. Fill the grid in from the top-left, and the bottom-right corner holds the answer.
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 Trip Budget Allocator, a pipeline you can run and download. See every Software Engineer project.
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