A puzzle invented by an American but popularized by the Japanese.
The puzzle consists of a 9x9 grid which is partitioned into nine 3x3 boxes. Typically, 20 to 25 of the 81 cells are pre-filled with one numerical digit. The objective is to fill the remaining cells such that each digit (1 through 9) is represented in every row, column and box.
Easy to moderate puzzles can be solved via simple elimination.
More difficult puzzles require some "what if" propositions to solve.
Solving Sudoku by hand is a routine diversion but writing a program to solve or create Sudoku puzzles is a more interesting challenge.
I got a Sudoku book for Christmas and wrote a program that could solve any of the puzzles in under a second.
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