RULES: The objective of sudoku is to enter a digit from 1 through 9 in each cell, in such a way that: Each horizontal row (shown in pink) contains each digit exactly once Each vertical column (shown in yellow) contains each digit exactly once Each subgrid or region (shown in green) contains each digit exactly once In each sudoku puzzle, several digits have already been entered (the "givens"); these may not be changed. The puzzler's job is to fill the remainder of the grid with digits –respecting, of course, the three constraints mentioned earlier.