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"It Tries Every Grid"

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State

A Sudoku solver does not try grids — nine candidates in eighty-one cells make a number with seventy-eight digits, and no machine tries that.

Show

Each clue is a collapsed domain; each row, column, and box is a cluster of difference constraints; propagation deletes candidates by the thousands.

Watch for

The solver's speed is not the speed of trying — it is the speed of ruling out.

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