Sudoku End Game Tactic 1 - Gwendolin's Thread

 

Here is one end game tactic which I use pretty successfully after correctly filling in 80% of the blank spaces in the 9 x 9 sudoku game. Its called Gwendolin's thread. Gwendolin was a little girl who went into the forest to pick some flowers. As she went deeper and deeper, she realised she was lost. 

Rather than panic, she used this tactic whereby she pulled out a spool of thread (OK, I know its far fetched, who goes into the forest with a string of thread, but you get my point right ? Its just a fairy tale !).

So she walks with the thread, ties one end at a random tree, and walks in the direction where she thought she came from. The moral of the story, if she came to another junction and there was a thread, she would quickly realise that she was walking in circles ! So she would backtrack and where there was no thread, she would then continue in the opposite direction until she made her way out of the forest. OK its just a fictional fairy tale but the concept is to use markers, and thread so that you know if you had made a wrong choice, you can BACKTRACK your move and try the opposite direction or use the alternate number.

For the Sudoku game, if the square is a 50 / 50 (50% either a certain number or the other), then I will use shortform LETTERS in place of numbers and play along.

So in the game above, I have about 18 unfilled squares, an then I can choose ar an unfilled square to use either (for example) a 9 or a 7. I would proceed with 9 and then write in that box the number N. This is the arbitrary number which I have taken as a 50 : 50. Each Uncertain Number I will put in the 50% chance of that number (a guess)  in LETTER format. 

Therefore 

1 : O

2 : D

3 : T

4 : F

5 : V

6 : X

7 : S

8 : E

9 : N.

Try it. If there happens to be to of the same letters on the same line or column you realise you have the wrong path (say 2 Os)  So I will backtrack and change ONLY THE LETTERS from when you started. The key is note down which square when you put in the first letter, and once you encounter a wrong path, backtrack and replace the letter with the other possible letter.

So, the rule of thumb should be, for me anyway, is when I have a 50 % chance it could either be the number One or 50% of the number Nine. 

I will start with using One, or O and play on. If I reach the end of the game with no duplicate, essentially I have WON the game. I will then replace the letters with the number and that is the end game.

if I reach a line where there is 2 of the same letters, I will backtrack to the first square where I put the O, and delete that, and add in the other possible number, say Nine or N.

If my earlier steps were all correct, that should lead me to the path of winning. 

This is called Gwendolin's Thread.

Enjoy your Sudoku readers !