In search of the quickest way to solve Sudoku puzzles
Once you can solve “Expert” sudoku puzzles, you’re almost certainly an addict (otherwise you wouldn’t have become an expert). Then the challenge is the race against time. With this in mind, I’ve been searching for quicker ways to reach the solution and here’s what I’ve found –
I’m assuming that you’re an expert already, so won’t set out the tips for solving the puzzles. These are readily available via Google. but nowhere have I found speed techniques.
First is your choice of App. Find one that makes your scanning and searching quick and easy. My favourite is Sudoku – Free Classic Sudoku by Dailyinnovation.biz I will assume that your choice has the same features.
Don’t waste your time looking for unique candidates such as numbers that can only fit in one particular cell of a row, box or column.
Start off by switching on the “Highlight areas” option in Setup and then immediately fill in all possible candidates in every cell. The app will provide you with the means of filling in these candidates.
It is critical that you get this right as a mistake now will cost you the game. However, the right App will not allow you enter a wrong number, so this is quick and easy and I no longer re-check my entries as I used to.
Swtich off “Highlight areas” – you no longer need it and it muddies the water.
Next, a very quick scan for double pairs like 2 7 twice in one box, column or row. Also, you should be able to easily spot the triples, like 2 3, 2 7 and 2 3 7. When you find a pair of pairs or set of triples, eliminate the remaining 2’s, 3’s and 7’s (in this example) in that same box, column or row as the case may be. You’ll also pick up any unique candidates in any box, column or row as you go along. That’s dealt with the quick and dirty.
Be careful. Always work with “Pencil on” unless you have a unique candidate. Otherwise you can accidentally enter a wrong number. If that happens, Undo immediately.
Next comes a fairly quick and fairly dirty search. Remember this is all about speed, so, as in “How to pass exams“, we want to pick up the easy points first.
Check each row. You’re looking for any candidates of that row that are limited to a particular box like the green 6s, in this example. If you find an example, eliminate that candidate from all other rows within the same box.
At the same time, check for any box that has a particular candidate in only one row or column within that box, like the green 3’s in this example.
Eliminate that candidate from the rest of the row or column.
And, because you’re an expert, you’ll have no trouble checking for naked subsets at the same time. Eliminate the orange 3s and 4s and the 2s and 4s respectively in the example below.
Now speed check the columns for similar results as the above rows. You will get to the stage where you can check rows, boxes and columns all in one hit.
By now you should be well on the way to the solution. Don’t waste time repeating the above, unless something obvious jumps out at you.
Touch a number 1 on the puzzle. All 1 candidates will be highlighted. Now, repeat the above searches but specifically focusing on 1. But now you’ll include searches for hidden subsets, like the 7 and 3 in the example, eliminating the remaining candidates in their cells and, in this case, leaving a unique 9 in row 4. Highlighting the 1’s makes this easier.
Repeat 2 through 9, then again 1 through 9, until you’re stuck, but you’ll usually have solved the puzzle by now.
If not yet solved, it’s time to slow down and bring out the big guns – Unique rectangle, X-wing, Swordfish (also known as XY-wing), and (heaven forbid) Forcing chain. Find one of those and you should be racing to a solution, usually using quick-scans again.
If you finally have to give up and take a clue, choose a cell with a number that appears most in the puzzle and which has the most other candidates in the cell.
Good luck and good speed!