A simple sudoku solver. Reads a grid from standard input and prints the solution on standard output.

The input should match ^[1-9:]{81} (81 digits or colons). Each character represent the content of one cell of the grid, in Z-order; colon means “empty”. See the sudoku.in file for an example.

The output is pretty-printed using UTF-8 box-drawing characters. When using sudoku.in as the input file, it should look like this:

╔═╤═╤═╦═╤═╤═╦═╤═╤═╗
║1┃2┃4║3┃8┃6║9┃7┃5║
╟━╋━╋━╫━╋━╋━╫━╋━╋━╢
║9┃5┃3║7┃1┃2║8┃4┃6║
╟━╋━╋━╫━╋━╋━╫━╋━╋━╢
║7┃6┃8║5┃4┃9║1┃2┃3║
╠═╪═╪═╬═╪═╪═╬═╪═╪═╣
║6┃7┃5║2┃9┃4║3┃8┃1║
╟━╋━╋━╫━╋━╋━╫━╋━╋━╢
║4┃3┃9║8┃5┃1║7┃6┃2║
╟━╋━╋━╫━╋━╋━╫━╋━╋━╢
║8┃1┃2║6┃7┃3║5┃9┃4║
╠═╪═╪═╬═╪═╪═╬═╪═╪═╣
║3┃9┃1║4┃6┃7║2┃5┃8║
╟━╋━╋━╫━╋━╋━╫━╋━╋━╢
║2┃8┃6║9┃3┃5║4┃1┃7║
╟━╋━╋━╫━╋━╋━╫━╋━╋━╢
║5┃4┃7║1┃2┃8║6┃3┃9║
╚═╧═╧═╩═╧═╧═╩═╧═╧═╝

If it doesn’t display correctly on your computer, either your software or your fonts don’t properly support UTF-8 (e.g. Courier has issues with box-drawing characters).

To compile and run the code with the test input sudoku.in, simply run:

make

sudoku.in is supposedly a list of the 95 hardest sudoku grids in existence. It was stolen from dos486.