Sudoku originally called Number Place) is a logic-based, combinatorial number-placement puzzle. The objective is to fill a 9×9 grid with digits so that each column, each row, and each of the nine 3×3 subgrids that compose the grid (also called "boxes", "blocks", or "regions") contain all of the digits from 1 to 9. The puzzle setter provides a partially completed grid, which for a well-posed puzzle has a single solution.
Completed games are always an example of a Latin square which include an additional constraint on the contents of individual regions. For example, the same single integer may not appear twice in the same row, column, or any of the nine 3×3 subregions of the 9×9 playing board.
How to play
1) Crosshatching (shown in the first picture). You scan rows and columns to eliminate where a specific number can be in a given region.As you can see there is only one legal place left for the 1(marked green).
2)Counting. In counting you simply count all the different numbers that's in a row,column and region that connects to one cell. if there is just one number missing then thats what should be in the cell. Take a look at the second picture and see if you can figure out the missing number in the green cell.
