Sudoku is a one-rule puzzle game that can be either satisfyingly simple or deceptively difficult.
Cells where an individual number goes are called squares; squares are then sectioned off in groups of nine to create boxes.They create a three-box-by-three-box grid.
To solve a sudoku puzzle, each row of nine squares must contain the numbers 1 through 9. Each column must also contain the numbers 1 through 9, and each box must contain the numbers 1 through 9. No row, column, or box may repeat any number.
Many squares will already be filled in, no matter the puzzle level. Based on those filled in squares, using logic and the process of elimination, begin deducing which numbers fit in the empty squares.
Use cross-hatching, the process of figuring out where a number fits by eliminating possibilities based on numbers in the other squares in the same row, column, and box. For example, if the top-middle box contains no number 8, and the other boxes along the top row of the sudoku board contain 8s in the first and second rows, process of elimination dictates the 8 in the upper-middle box must be in the third row.
Once you've solved one square by cross-hatching, use the technique to fill in as many squares in that single box as you can.
Once you have exhausted one box, either by filling it in completely or running out of discoverable solutions, continue to the next box, then the next box, until you've gone through all nine boxes.
Once you've gone through all the boxes, return to the box you started with and repeat the process, as it's likely there are now more available solutions.
Look at a single square, and count 1 through 9 through all possibilities, marking off ones that are immediately disqualified because it already appears either in the same box, the same column, or in the same row as the empty square.