Sudoku is a logic puzzle set on a nine by nine grid. The goal is to fill in the blank spaces in the puzzle with the digits 1 thru 9, according to these four rules:
each row contains each digit once;
each column contains each digit once;
each three by three sub-grid (with dark lines) contains each digit once;
each cell contains only a single number.
To solve the puzzles, you must deduce what numbers go where, based on the positions of numbers already in the puzzle. As an example, here is a puzzle:
Consider the left-most column. Since it has the numbers 4, 5, 6, 7, and 8, we know that the blank spaces in that column must be filled with 1, 2, 3, and 9. Now, look at each row that intersects those blank spaces. The first one has a 9 already, as do the last two. So, the only possible place to put a 9 is in the starred cell:
In that same puzzle, look at this starred cell:
We know from the right-most column that that cell cannot have a 1, 3, 5, 6, or 9. But, since the bottom-right 3x3 grid cannot duplicate any numbers either, we know that that cell cannot be 2, 7, or 8. So, that cell must be a 4.
