Solving sudoku

SOLVING SUDOKU by Michael Mepham
What is sudoku? About guessing
You would imagine that with such a name this puzzle originated in Japan, but Try not to. Until you have progressed to the tough and diabolical puzzles,
it has been around for many years in the UK. However, the Japanese found an guessing is not only totally unnecessary, but will lead you up paths that can
example under the title ‘Number Place’ in an American magazine and translated make the puzzle virtually unsolvable. Simple logic is all that is required for gentle
it as something quite different: su meaning number; doku which translates as and moderate puzzles. Only a small proportion of my sudokus will require deep
single or bachelor. It immediately caught on in Japan, where number puzzles analysis, and that is dealt with later on in this introduction.
are much more prevalent than word puzzles. Crosswords don’t work well in the
Japanese language. Making a start
The sudoku puzzle reached craze status in Japan in 2004 and the craze spread 1
to the UK through the puzzle pages of national newspapers. The Daily Telegraph
To solve sudoku puzzles you will
uses the name Sudoku, but you may see it called su doku elsewhere. However, 8 3 4 use logic. You will ask yourself
there is no doubt that the word has been adopted into modern parlance, much
questions like: ‘if a 1 is in this box,
like ‘crossword’. 3 4 8 2 1 will it go in this column?’ or ‘if a 9
Sudoku is not a mathematical or arithmetical puzzle. It works just as well if the 7 is already in this row, can a 9 go
numbers are substituted with letters or some other symbols, but numbers work in this square?’
best. 9 4 1 8 3 To make a start, look at each of
The challenge the boxes and see which squares
8 3 4 Here is an unsolved sudoku
are empty, at the same time
4 6 5 7 1 checking that square’s column
3 4 8 2 1 puzzle. It consists of a 9x9 grid
and row for a missing number.
that has been subdivided into 7

© Crosswords Ltd, 2005
7 9 smaller grids of 3x3 squares. In this example, look at box 9.
1 2 5 3 9 There is no 8 in the box, but there
9 4 1 8 3 Each puzzle has To solveand
a logical
is an 8 in column 7 and in column
unique solution. the 7 2 4 8

#5096
puzzle, each row, column and 8. The only place for an 8 is in
box must contain each of the column 9, and in this box the only
4 6 5 7 1 numbers 1 to 9. square available is in row 9. So
put an 8 in that square. You have
7 Throughout this document I refer

solved your ﬁrst number.
to the whole puzzle as the grid, a
1 2 5 3 9 small 3x3 grid as a box and the
cell that contains the number as
7 2 4
#5096

a square.
Rows and columns are referred to with row number ﬁrst, followed by the column
number: 4,5 is row 4, column 5; 2,8 is row 2, column 8. Boxes are numbered 1−9
in reading order, i.e. 123, 456, 789.

Copyright © 2005 Crosswords Ltd

8 3 4 2 8 3 4 4
Continuing to think about 8, There is a similar situation with
3 4 8 2 1 there is no 8 in box 1, but you 3 4 8 2 1 the 4s in boxes 4 and 5, but here
8 8 can see an 8 in rows 1 and 2. 8 8 the outcome is not so deﬁnite.
7 So, in box 1, an 8 can only go in
7 Together with the 4 in column
9 4 1 8 3 row 3, but there are 2 squares 9 4 1 8 3 7 these 4s eliminate all the
available. Make a note of this 4 4 available squares in box 6 apart
by pencilling in a small 8 in both from two. Pencil a small 4 in
squares. Later, when we have these two squares. Later on, one
4 6 5 7 1 found the position of the 8 in
4 6 5 7 1 or other of your pencil marks will
7 boxes 4 or 7 we will be able to 2 7 be proved or disproved.


disprove one of our 8s in box 1.
1 2 5 3 9 1 2 5 3 9
7 2 4 8 7 2 4 8
#5096

#5096
8 3 4 3 8 3 4 5
We were looking at box 9. As you
3 4 8 2 1 can see, there is a 2 in boxes 7 3 4 8 2 1 Having proved the 2 in box 9
8 8 and 8, but none in box 9. The 2s 8 8 earlier, check to see if this helps
7 in row 8 and row 9 mean the only
7 you to solve anything else. For
example, the 2 in box 3 shows
9 4 1 8 3 place for a 2 in box 9 appears 9 4 1 8 3 where the 2 should go in box 6:
to be in row 7, and as there is 4 24
already a 2 in column 8, there is it can only go in column 9, where
only one square left in that box 2
there are two available squares.
4 6 5 7 1 for a 2 to go. You can enter the 2
4 6 5 7 1 As we have not yet proved the
2 7 for box 9 at 7,7. 2 7 position of the 4, one of the

squares may be either a 4 or a 2.
1 2 5 3 9 1 2 5 3 9
7 2 4 8 7 2 4 8
#5096

#5096

8 3 4 6 The search for the lone number
Now solve a number on your Daily Telegraph easy sudoku puzzles are called ‘gentle’. This indicates a level of
3 4 8 2 1 own. Look at box 8 and see complexity that can be tackled by beginners and casual sudoku solvers. However,
8 8 where the number 7 should go. no matter what level of puzzle you are attempting, there are a few strategies
7 that will allow you to get to a solution more quickly.
Continue to solve the more
9 4 1 8 3 obvious numbers. There will The key strategy is to look for
4 24 come a point when you will 3 5 678 4 1 the lone number. In this example,
need to change your strategy. all the options for box 5 have
2 7 2 5
4 6 5 7 1 What follows will provide you
178
been pencilled in. There are three
with some schemes to solve the 5 2 6 squares where the number 1
2 7

complete sudoku. might go, but look between the
The best way to learn how to 8 6 5 9 3 8 and 3 − there is a lone number
1 2 5 3 9 127 17 1. It was not otherwise obvious
solve sudoku is to practice. So 3 4 8
7 2 4 8 I’ve put this sudoku below. See that the only square for the
#5096
1469 149 1 49 1469
how far you can get using the 5 8 3 7 number 1 was row 6, column 5,
strategies I’ve discussed so far. 1678 as there is no number 1 in the
5 3

immediate vicinity. Checking the
6 9 5 1 adjacent boxes and relevant row
and column would not provide an
1678
9 4 2 5 immediate answer either − but no

#5335
other number can go in that box.
8 3 4 While our example uses pencil
3 5 678 4 1 marks to illustrate the rule, more
3 4 8 2 1 7 2 5 experienced solvers are quite
capable of doing this in their
7 5 2
78
6 head.

9 4 1 8 3 8 6 5 9 3 Remember that this principle is
true for boxes, rows and columns:
27 7
3 4 8 if there is only one place for a
number to go, then it is true for
469 49
5 8 1 3 7 49 469 that box, and also the row and
4 6 5 7 1 5 678
3 column it is in. You can eliminate

all the other pencilled 1s in the
7 6 9 5 1 box, row and column.

678
9 4 2 5

#5335
1 2 5 3 9
7 2 4
#5096


Twins Triplets
Why limit yourself to one when sometimes two can do the job? In sudoku we In the previous example, our solver’s twins did just as well as a solved number
can easily become blind to the obvious. You might look at a box and think there in helping to find her number. But if two unsolved squares can help you on your
is no way of proving a number because it could go in more than one square, way, three solved numbers together certainly can.
but there are times when the answer is staring you right in the face. Take the Look at the sequence 2−8−1 in row 8. It can help you solve the 7 in box 8. The
sudoku opposite. It’s an example 7s in columns 5 and 6 place the
5 4 9 7 2 of a gentle puzzle. The solver 4 6 7s in box 8 at either 8,4 or 9,4. It
has made a good start at finding is the 7 in row 7 that will provide
2 7 9 3 6 4 the more obvious numbers, but 3 8 6 sufficient clues to make a choice.
9
8 7 4 having just solved the 9 in box 4
she’s thinking about solving the 9 3 9 7 2 Because there can be no more
7s in row 7, the 2−8−1 in row
1 9 4 8 7 in box 1. It seems impossible, with 1 8 9 7 8 forces the 7 in box 7 to be in
just a 9 in row 1 and another in
7 5 4 9 column 2 that immediately affect 9
row 9. Although you don’t know
1 which square it will be in, the
4 7 9 2 1 box 1. 5 3 7 2 unsolved trio will prove that no
But look more carefully and more 7s will go in row 9, putting
4 6 3 you’ll see that the 9 in row 8 6 8 4 7 our 7 in box 8 at row 8. A solved
2 9 3 4 7 precludes anythe 9row 8 of box
7. In addition,
9 in
in column 2 2 8 1
7 row or column of three squares
in a box is good news. Try the
7 7 7 7
3 1 4 6 eliminates the square to the right
5 2 same trick with the 3−8−6 in row
of the 4 in that box, leaving just 2 to see if this triplet helps to
the two squares above and below solve any more.
5 4 9 7 2 the 2 in box 7 available for the 9.
You’ve found a twin!
2 7 3 6 4 Pencil in these 9s. While you
9 8 7 4 don’t know which of these two Eliminate the extraneous
will end up as 9 in this box, what We have looked at the basic number-finding strategies, but what if these are
1 9 4 8 7 you do know is that the 9 has just not up to the job? Until now we have been casually pencilling in possible
7 5 4 9 tocannot go in column 3 of box
9
be in column 3. Therefore a numbers, but there are many puzzles that will require you to be totally
methodical in order to seek out and eliminate extraneous numbers.
4 7 9 2 1 1, leaving it the one available If you have come to a point where obvious clues have dried up, before moving
square in column 1.
4 69
3 into unknown territory and beginning bifurcation (more on that later), you should
ensure that you have actually found all the numbers you can. The first step
2 9 3 4 7 towards achieving this is to pencil in all possible numbers in each square. It takes
9 less time than you’d think to rattle off ‘can 1 go’, ‘can 2 go’, ‘can 3 go’, etc., while
3 1 4 6 checking for these numbers in the square’s box, row and column.
Now you should look for matching pairs or trios of numbers in each column,
row and box. You’ve seen matching pairs before: two squares in the same row,
column or box that share a pair of numbers.

You can see what I mean in this Stepping up the sudoku action
illustration. In this row at column The strategies discussed so far will allow you to solve all but the hardest sudoku.
1 there is a 1 8 and at column 6 However, when you come to the more testing grades of tough and diabolical
there is also a 1 8. This matching there are times when the logic required is much more difficult. What happens
pair is telling you that only either 1 or 8 is definitely at one or other of these in these puzzles is that you will reach a point where there are apparently no
locations. If that is true then neither of these numbers can be at any other numbers that can be solved.
location in that row. So you can eliminate the 1 and 8 in any other square of the
row where they do not appear together. As you can see, this immediately solves Let me emphasise at this juncture that the sudoku puzzles with these
the square at column 5. This rule characteristics represent only a very small minority of puzzles. They will
can be applied to a row, column be among the diabolical and possibly the tough puzzles. They are valid puzzles,
or box. and many advanced sudoku solvers have devised logic schemes (and computer
programs) for solving them.
The number-sharing rule can be taken a stage further. Say you have three
squares in a row that share the numbers 3, 7 and 9 and only those numbers. However, the wonderful thing about sudoku puzzles is that you don’t have to be
They may look like 3 7, 3 9, 7 9 or 3 7, 3 9, 3 7 9 or even 3 7 9, 3 7 9, 3 7 9. a genius or a computer programmer to solve even the most diabolical example.
In the same way as our pairs example worked, you can eliminate all other If you are meticulous and patient and have mastered the gentle and
occurrences of those numbers anywhere else on that row (or column or square). moderate puzzles then you can solve every Daily Telegraph puzzle.
It will probably take a minute or so to get your head round this one, but like the There is a logical and unique solution to each and every one. Although there are
pairs, where we were looking for two squares that held the same two numbers some computer programs that cry foul when they are incapable of solving some
exclusively, here we are looking for three squares that contain three numbers sudokus, this does not mean that you will not be able to solve them with care
exclusively. and by the systematic elimination of alternatives. Hundreds (if not thousands) of
Sometimes, the obvious simply needs to be stated, as in the case of two squares Daily Telegraph readers, just ordinary, intelligent people, return correct solutions
that contain 3 7 and 3 7 9. If the 3 and the 7 occur only in those two squares to the most difficult sudokus published in the newspaper’s daily competition.
in a row, column or box, then either the 3 or 7 must be true in either one of the When all else fails
squares. So why is the 9 still in that square with what is so obviously a matching
So how do they do it? I’ll wager that when you started sudoku you didn’t think
pair? Once that 9 has been eliminated, the pair matches and can now eliminate
you’d be dealing with methodological analysis and bifurcation, but these are the
other 3s and 7s in the row, column or box. You could say this was a ‘hidden’ pair.
technical terms for the process of picking a likely pair of numbers, choosing
You may find such hidden pairs in rows, columns or boxes, but when you find one and seeing where the number you have chosen gets you. Because you can
one in a box, only when it has been converted to a true matching pair can you be confident that one of the numbers will eventually produce a route to the
consider it as part of a row or column. Hidden trios work in exactly the same solution, it is simply a matter of carefully analysing the options and testing your
way, but are just more difficult to spot. Once you have assimilated the principle choice. If your first choice doesn’t work out then you take the alternative route.
of two numbers sharing two squares exclusively or three numbers sharing
I must emphasize that this final strategy is reserved for the most difficult
three squares exclusively you will be well on the way to solving the most difficult
of the diabolical and, occasionally, tough puzzles – when all else fails.
sudokus.
Think of a sudoku puzzle as a maze. Gentle and moderate puzzles have a simple
path straight through to the exit. Tough and diabolical puzzles may have dead-
ends which force you to try different routes. A tough puzzle may only have one
of these dead ends to cope with, or it may simply have a more tortuous route to
a solution. Diabolical puzzles will have at least one, and maybe more paths that
you could follow before finding the number that leads to a logical exit.


The way to navigate this maze can be found in classical mythology, so allow me Tough sudokus
tell you a story. In tough and diabolical sudokus there is still a beginning and an end to the maze,
Ariadne’s thread but now the illustration is slightly more complicated:
As well as creating sudoku puzzles, I also compile the giant general knowledge a2
crossword for The Daily Telegraph weekend supplement, so excuse me if I put a1
that hat on for a few moments to remind you briefly of the story of Ariadne’s
thread.
Ariadne was the daughter of King Minos of Crete, who conquered the Athenian b1
nation. An unfortunate intimacy between Ariadne’s mother and a bull resulted b2
in the birth of the monster − half-bull, half-man − called the Minotaur, who was
banished to spend his days in the Labyrinth. King Minos, being something of a c1
tyrant, called for tribute from Athens in the form of young men and women to be
sacrificed to the Minotaur.
Somewhere along your route you will find that none of your pencilled numbers
The young Athenian hero, Theseus, offered to accompany a group of the young provide a next step. You may look at a square at a1 where there are two options.
unfortunates into the Labyrinth so that he could kill the Minotaur and save One of the options takes you up a blind alley at a2 and you rewind your ‘thread’
Athens from the cruel tribute. Ariadne fell in love with Theseus and, not wishing to a1 and choose the other number that takes you to a solution.
to see him lost in the Labyrinth once he had dealt with her bovine half-brother,
There may be another pair at b1 that you could have chosen. One of the choices
she provided him with a means of escape − a silken thread. Theseus had simply
would be wrong and would take you to point b2, but the other would have been
to unwind it while he went through the Labyrinth; should he come to a dead end
correct. Although the route is different, you end up with a solution.
he could rewind it to the point where he had made a choice of paths and continue
his search using the alternative route. The scheme worked out beautifully, the However, if you had chosen either of the third pair at c1, both numbers would fail
Minotaur was slain, Theseus found his way back out of the Labyrinth and Ariadne to provide sufficient clues to get to the end.
. . . well, she got her ball of string back, no doubt. At a dead end you may be presented with numerous choices of pairs. Although
Replacing my sudoku hat, I hope that this tale of Ariadne’s thread has served to one or other of each of these pairs will be correct in the final solution, there is no
illustrate the method used to solve tough and diabolical sudokus. Take a look at guarantee that it will provide sufficient clues to take you through to the end of
the following illustration that represents a gentle or moderate sudoku. the puzzle.
In a 9×9 tough sudoku the chances are that your first or second choice will
get you on the right track.
Your labyrinth has been straightened out in the diagram. It may not feel like it at
the time, but there’s just a start and finish.


Solving a diabolical puzzle with So our solver returns to 1,5 and
8 4 5 699 1 7 2 tries the 9. Now we are able to
Ariadne’s thread
In its structure there is little difference between a tough sudoku and a diabolical 9 4 8 1 5 prove the 9 at 2,1, but nothing
else is obvious; every square is
puzzle. The difference is that there are more places where clues can run out and
more apparent dead ends.
1 5 3 2 4 9 8 left with options. In this case we
8 4 5 69 1 7 2 Take the example illustrated 9 8 5 1
could leave both 9s, because we
proved without doubt that the
4 8 1 5 here: you can see the squares 8 1 5 9 6 at 1,5 could not be correct,
our solver has managed to but if the 6 had simply left us
1 5 3 2 4 9 8 complete using the strategies we 4 6 5 9 1 3 2 8 7 without sufficient clues, as the
have discussed previously. For 9 did, we wouldn’t know which
9 8 5 1 clarity’s sake we’ll ignore all the 7 4 3 1 8 5 9 2 6 was true. So, rather than start a
8 1 5 9 ‘pencil’ first pair: at 1,5 weexcept
marks on the grid
5 9 4 6 1 3 new, uncertain path it is better to
for the have return to the situation we were in
4 6 5 9 1 3 2 8 7 either a 6 or a 9. This would be 1 3 9 5 4 before we chose at 1,5 and find
the equivalent to point a1, b1 or c1
7 4 3 1 8 5 9 2 6 on our tough sudoku illustration. another square to try from. This
is a base we know to be true.
5 9 4 6 1 3 There is atcould one other pair
our solver
least
have chosen on
1 3 9 5 4 the grid, but this was the first,
so let’s be logical and use that.
Our solver chooses to try the 6
9 8 4 5 69 1 3 7 2 first, and the following diagram
6
shows the numbers she is able 6 8 4 5 9 1 36 7 2
3 In this illustration we can see that
2 3 7 4 9 8 1 6 5 to complete using this number in our solver looks at square 1,7
9 3 2 4 7 8 1 6 5 where the choice is between a 3
1 5 6 7 3 2 4 8 9 bold grey. and a 6. Choosing the 3 she finds
1 5 7 6 3 2 4 9 8 herself on a path that takes her
3 7 9 8 2 4 5 1 But with just two squares to fill, to just two more to go . . .
3 2 9 8 6 7 5 4 1
8 2 1 5 7 6 3 9 look at what we have: at 4,8 the Whoops. We need a 2 to
box needs a 4 to complete it, but 8 7 1 2 5 4 6 3 9 complete box 7, but there’s
4 6 5 9 8 7 1 3 2 there is already a 4 in that row already a 2 in that row at 9,5. In
at 4,6. Similarly, at 5,4 that box 4 6 5 9 1 3 2 8 7
7 4 3 1 2 6 8 5 9 needs a 6, but one already exists
box 9 we need an 8, but there’s

1 3 7 4 3 1 8 5 9 2 6 an 8 in row 8 already. It has
5 9 8 2 4 6 7 in that row at 5,7. No second happened again!
guess was needed to prove that
5 9 8 7 4 6 1 3
6 1 2 3 7 9 8 5 4 at 1,5 the 6 was incorrect.
1 6 3 2 9 7 5 4


36 At this point, I find a couple of Acknowlegements
8 4 5 1 6 7 2 slugs of scotch help. Since the first Daily Telegraph Sudoku book was published in May many new
4 8 1 5 If you still have the time or solvers have contacted me through the website dedicated to Daily (and now
inclination, wind in the thread to
1 5 3 2 4 9 8 get back to 1,7. 3 was chosen
Sunday) Telegraph sudoku at www.sudoku.org.uk, and I’d like to thank them
for their helpful suggestions and tips. I must also thank the thousands of Daily
last time. Try 6.
9 8 5 1 Telegraph readers who have contacted me since the puzzle first appeared in
I’ll let you finish up. I’m off down their newspaper. Their suggestions and unbounded enthusiasm for sudoku is
8 1 5 9 the pub! reflected in the size of my daily postbag. Many of their comments, questions and
4 6 5 9 1 3 2 8 7
suggestions have helped to shape this tutorial.
Michael Mepham
7 4 3 1 8 5 9 2 6 Frome, Somerset, 2005
5 9 4 6 1 3
1 3 9 5 4

The last word
Every day I receive a few emails from Daily Telegraph solvers who tell me how
they struggled on Tuesday’s moderate puzzle, but flew through the diabolical
example on Friday. My answer is always that my grading of puzzles is subjective.
I have no way of knowing what mistakes a solver might make, how experienced
he or she is or whether he or she is suffering from a bad day or not. The same
puzzle may take one person thirty minutes and another two hours, and it’s not
always down to the level of experience of the solver. Also, some people have an
innate ability to spot the clues. With practice, others develop this ability without
realising it.
No matter how good or bad you are at sudoku, what I can guarantee is that these
puzzles will give you a good mental workout. As keep fit for the brain, in my
experience as a puzzle setter, sudoku is as good as it gets. Have fun.

Now try some examples...

Sample Sudoku
GENTLE TOUGH
1 4 2 5 Easily solved with simple
1 9 8 Should give an experienced
2 7 1 3 9 logic. The difference between
these and Moderate puzzles
6 8 5 3 solver some entertainment.

4 may sometimes seem
marginal. Ratings must be
7 6 1
2 7 1 6 subjective, as there is no way
of knowing the skill level of
3 4 9
4 the solver. Ease of solving 5 4
depend on your skill levels,
6 7 4 3 so even a gentle puzzle may 1 4 2
feel tough to a beginner. But
7 don’t worry, after the ﬁrst 5 7 9
few numbers have been
1 2 7 3 5 found you’ll be solving like a 1 8 4 7
veteran in no time.
3 8 2 7 7 9 2
#5232

MODERATE DIABOLICAL
6 2 1 Will require some skill and
9 7 8 6 Be prepared for multiple
8 7 1 practice. 3 1 5 2 seeming dead ends that
require advanced solving
1 7 3 2 8 6 strategies or intelligent
guessing. This example is as
7 3 4 7 5 6 hard as you will ever get in a
newspaper or book, but they
5 8 3 7 can be much more difﬁcult.

8 4 7 5 1 7
4 6 5 8 1 9

1 7 4 2 6 3 5
4 6 5 5 4 8 7
#5329


Solutions
GENTLE TOUGH
8 1 3 4 2 9 7 6 5 3 4 1 9 2 7 5 6 8
4 6 2 5 7 1 8 3 9 6 9 2 1 8 5 7 3 4
7 9 5 3 6 8 1 4 2 8 5 7 4 6 3 1 9 2
2 4 7 1 5 3 9 8 6 1 3 4 2 9 6 8 7 5
5 3 9 8 4 6 2 1 7 2 7 8 5 3 4 6 1 9
6 8 1 2 9 7 4 5 3 5 6 9 7 1 8 4 2 3
9 7 8 6 1 5 3 2 4 4 2 5 3 7 1 9 8 6
1 2 6 7 3 4 5 9 8 9 1 6 8 4 2 3 5 7
3 5 4 9 8 2 6 7 1 7 8 3 6 5 9 2 4 1

MODERATE DIABOLICAL
7 3 6 2 9 1 4 5 8 2 9 5 7 4 3 8 6 1
8 4 2 6 5 3 9 7 1 4 3 1 8 6 5 9 2 7
5 9 1 7 8 4 6 3 2 8 7 6 1 9 2 5 4 3
9 1 7 5 3 8 2 4 6 3 8 7 4 5 9 2 1 6
2 5 4 9 6 7 1 8 3 6 1 2 3 8 7 4 9 5
6 8 3 1 4 2 7 9 5 5 4 9 2 1 6 7 3 8
4 6 9 3 1 5 8 2 7 7 6 3 5 2 4 1 8 9
1 7 5 8 2 9 3 6 4 9 2 8 6 7 1 3 5 4
3 2 8 4 7 6 5 1 9 1 5 4 9 3 8 6 7 2

sudoku worksheet sudoku worksheet


Solving sudoku

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