Design a crossword puzzle solver system

## Design a crossword puzzle solver Design a system that can **solve a crossword puzzle**. ### Inputs - A 2D `board` grid: - Some cells are **blocked** (black squares) - Some cells are **empty** (need letters) - Optionally, some cells may already contain a fixed letter - A `dictionary` (word list) A valid solution must: - Fill every non-blocked cell with a letter - Every horizontal and vertical contiguous run of non-blocked cells (a “slot”) must form a word present in the dictionary - Respect pre-filled letters and intersections ### Outputs - One solved board (or all solutions / top-K solutions, if you choose—state your choice) - If no solution exists, return “no solution” ### What to cover - Core algorithmic approach and data structures - How you prune the search space and handle constraints from intersections - Complexity considerations - How you would structure this as a service/API if the dictionary and traffic are large (caching, precomputation, scaling) State any assumptions (board size, dictionary size, allowed alphabets, whether repeated words are allowed, etc.).