This is a commandline version of the sudoku board game.
The sudoku board game is played on a 9x9 grid, divided into rows, columns,
and 9 blocks of 3x3 squares. The objective is to fill the empty squares
with the digits 1-9, so that each row, column, and block contains each
of the digits 1-9 (and hence, it is not possible for any digit to
appear twice in the same row, column or block).