Sudoku

I've recently learned how to play Sudoku. Below is my first hard Sudoku.

My first difficult Sudoku     3   2   1           4   6   5   7       8       9   9       8   7           4           1   2       9   6       5       4   2 4 3   1           8   7   6

Tina was able to solve the one above without any backtracking so I'll have to revisit it yet again! Another interesting thing she said which has rekindled my interest in Sudoku is that you should never have to use backtracking, there should always be a purely logical reason for every number to be filled in uniquely otherwise it defeats the purpose of Sudoku.

I revisited the one above yet again, and this time was able to complete it without requiring any difficult logic, so I must have not been meticulous enough writing in or checking the possibilities the first.

My first very hard Sudoku 2   6 3     1 7   4         6   5     3         2       8 9       7     2   5       9   1     7       8 5       2         4     7   6         8   8 1     3 7   2

I was unable to do this one and I had to get an online solver at scanraid.com to do it. The Scanraid solver is excellent because you can select which logic rules it's allowed to use to try and solve it. I was able to discover that this one required the simple coloring and y-wing. There are many other rules to be learned, the rules just mentioned are only tough, but there's also many diabolical and evil logical rules! So all in all I think this makes Sudoku very interesting, but requires a fair bit more learning yet ;-)

See also

• Scanraid Sudoku - An excellent solver allowing selection of rules used
• Sudoku9981 - Good explanation of logical rules
• DailySudoku - blank grids can be downloaded and printed from here