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Fourth level hard Sudoku puzzle 6, Strategies Techniques and Solution

Sudoku Hard Quick Solution and Easy Techniques level 4 Game 6

Solve and learn to solve Sudoku hard quickly by Sudoku hard strategy and techniques

Sudoku hard level 4 game 6 is solved in easy steps with early breakthroughs. Steps and the breakthroughs along with major Sudoku hard techniques explained.

Sections are,

  1. Sudoku hard level 4 game 6
  2. Solving Sudoku hard level 4 game 6 by Sudoku hard strategy and techniques
  3. Sudoku hard Strategy and techniques for easy solution
    1. What is a Cycle and how to use it in solving a Sudoku hard puzzle.
    2. How a single digit candidate valid cell is identified by Digit Subset Analysis (DSA) in solving a Sudoku hard puzzle.
    3. How digits possible for all empty cells (DSs) enumerated while solving a Sudoku hard puzzle.
    4. Single digit lockdown and its use in solving a Sudoku hard puzzle.
    5. Sudoku hard technique of double digit scan.

You may move directly to any of the above sections by clicking its link and return to previous position by clicking on browser back button.

While going through the solution you may click on say Cycle whenever it appears, to know how to form and use a Cycle and then return to the previous position to continue through the solution.

In the same way, for the other techniques also you may jump to details of a technique and after refreshing your concept return to the point from where you jumped.

Sudoku hard level 4 game 6

The following is the Sudoku puzzle that should engage your mind for some time. You should try to solve the game for better appreciation of the solution.

Sudoku hard level 4 game 6

The Rs are the row labels, Cs are the column labels and this we define as the stage 1 marked on top left corner.

This is a proper Sudoku hard and should pose a good challenge to experienced Sudoku players. For quick solution it needs keen attention to early breakthroughs by Sudoku hard patterns and techniques rather than following a conventional approach.

A note on the Sudoku game playing mode in solving the Sudoku hard game

Following our solution steps, the Sudoku hard game may be solved either with paper and pencil or better on a spreadsheet.


We prefer this approach because the pleasure in discovery of a breakthrough can be fully enjoyed this way.

The solution is broken up into stages for concentrating only on the present state of the game and easy backtracking in case of an error discovered at a later stage.

A note on cells colored with different colors conveying specific information

At any stage, the first cell found where a valid digit is entered, is colored TURQUOISE BLUE. All other Stage 2 valid cells are colored green. Third stage valid cells are colored blue, fourth stage valid cells are colored pink and the fifth stage blue green.

This helps to identify the cells identified as valid in the present stage.

We'll now solve the Sudoku hard game 6 and after the solution we'll explain in more details, the Strategy and techniques of solving a Sudoku hard game.

A system of internal links to these techniques sections and the solution section help you to navigate between the solution process and the detailed techniques used by clicking on the links.

Please spend your time fruitfully on the game trying to solve it before going through the solutions.

Let us solve our hard Sudoku puzzle now.

Solution to Sudoku hard level 4 game 6: Stage 1 to Stage 2

We'll show the puzzle board again for convenience of understanding.

Let's repeat, to follow the details accurately, you should better have the game actually with you written on paper, or better stillcreated in a spreadsheet.

Sudoku hard level 4 game 6

The first two valid cell easily identified are: R4C1 2 by scan R5,R6 -- R3C3 6 by scan R1,R2,C2. These are the only two valid cells obtained by easiest technique of row-column scans.

While searching for valid cells it feels good to have the early breakthrough in discovery of single digit 8 locked in R7C3 and R9C3 in C3 and bottom left 9 cell square. This is caused by scan for 8 in R8, C1.

First breakthrough: Single digit lock of 8 in R7C3, R9C3 by scan 8 in R8 and C1 both containing 8.

Because of this lock of 8 in C3, 8 cannot fill either R5C3 or R6C3, and is eliminated from DSs of these two cells. Effectively it amounts to C3 having 8 in bottom left square as well as in C3.

As a result, now we'll get a valid cell in R5C2 8 by scan 8 in C1 and locked 8 in C3.

Second breakthrough by a double digit row column scan: By digits [1,6] in both R4, C5 Cycle (1,6) formed in R6C4, R6C6. This is a double digit scan in a row and simultaneously in a column and is again an early breakthrough using a rarely used advanced technique.

This is the hallmark of Sudoku hard game,

Resistance in finding each valid cell right from the start and breakthroughs by advanced single digit lock technique and double digit scan techniques.

In central middle 9 cell square, possible digits are DS [3,9,4,7,8].

Again by the single column double digit scan for [3,9] in C5, we get the third breakthrough of two Cycles: Cycle (4,7,8) in C5 and Cycle (3,9) in R4 both in central middle 9 cell square.

Favorable outcome of the Cycles: With 3 in C4, R4C4 9 -- R4C6 3.

DS in R6C9 is [3,5,7,8]. But [3,5] in C9 cancel the two digits from the DS and results in [7,8] in R6C9. This in turn joins with [7,8] in R6C5 to form an important Cycle (7,8) in R6: Cycle (7,8) in R6.

This in turn creates two more Cycles: (3,5) and (7,9) in left middle 9 cell square.

Observe an additional Cycle (4,6,7) formed in R9C7, R9C8, R9C9.

You may verify the actions taken till now from the following second stage status.

Sudoku hard level-4 game 6 stage 2

Notice that we have evaluated the DS of a limited number of promising cells that too DS length restriced to 3. This strategic approach speeds up the process.

Solution to Sudoku hard level 4 game 6 Stage 3

R5C5 4 Cycle cancellation of 7 because of the Cycle (7,9) -- R5C9 1 by DSA: DS in three empty cells of R5 is [1,3,6] and [3,6] in R9 -- Cycle (3,6) in R5C7, R5C8 -- R8C7 1 scan R7,R9,C9.

By Cycle (4,7,8) in R9 a second Cycle (2,9) formed in R9.

With no immediate easy valid cells, short length (2 digit or maximum 3 digit long) DSs continued to be formed in the empty cells 9 cell square by 9 cell square.

All other 9 cell squares producing no breakthrough, attention finally turned on top left 9 cell square where an immediate breakthrough obtained.

BY DSA on C2 with DS [3,5,9] and 3 in both R1 and R3,

Cycle (5,9) in R1C2, R3C2 -- R8C2 3 cancel -- R1C1 4 by cancel of [2,3,7] from DS [2,3,4,7] in top left 9 cell square and R1C1 -- R7C1 9 cancel -- R5C1 7 cancel -- R5C3 9 cancel.

Now R2C1 3 by DS cancel of 5 from DS [3,5] -- R6C1 5 -- R6C3 3 -- R1C3 2 DS cancel of 7 -- R2C3 7.

Most hurdles and breakthroughs taken care of early in the Sudoku hard solution. Rest of the valid cells are easy to get.

For reducing clutter and ease of understanding let's close at this point and show you the status below.

Sudoku hard level 4 game 6 stage 3

Solution to Sudoku hard level 4 Game 6 Stage 4

DS in R1 and R1C4 [5,8,9]. With [5,9] in C4, R1C4 8 -- R1C7 5 with 9 in C7 -- R2C7 8 by scan 8 in R3,C8 -- R7C8 3 cancel 9 -- R7C7 2 -- R8C9 9 -- R3C7 4 cancel -- R2C8 9 cancel -- R3C9 2 exclusion.

Now we will get a valid digit R8C4 7 by scan of 7 in R7, Cycle (7,8) in R5, and 7 in C6.

With 2 in C6 and DS [2,4,5] in R8 and in R8C6, R8C6 DS gets [4,5] creating a Cycle (4,5) in R8.

This results in valid cell R8C5 2. Not very difficult, but not too easy also.

It follows with R2C5 5 by cancel of 2.

In R3, R3C4 1 by DSA cancel of [5,9] in C2 from [1,5,9] -- R3C6 9 by cancel.

Rest are very easy and shown next stage to reduce clutter.

The result of actions taken till now are shown below.

Sudoku hard level 4 game 6 stage 4

Solution to Sudoku hard level 4 game 6 Final Stage 5: Solved

Left over valid cells are by cancellation and occasional exclusion,

R3C2 5 by cancel -- R1C2 9.

In R2, R2C6 4 by cancel of 2 from DS [2,4] -- R2C4 2 cancel -- R8C6 5 -- R9C6 8 -- R9C3 5 -- R8C3 4 -- R7C3 8.

In R4, R4C7 7 by cancel of 4 -- R9C7 6 -- R9C8 4 -- R9C9 7 -- R6C9 8 -- R4C9 4 -- R4C5 8 -- R6C5 7.

With 1 in C4, R6C4 6 -- R6C6 1.

With 6 in C7, R5C7 3 -- R5C8 6.

R7C4 4, R7C6 6 by exclusion.

The final solved puzzle board is shown below.

Sudoku hard level 4 game 6 stage 5 final solution

Check for the validity of the solution if you need.

Overall strategy adopted and techniques used for solving a Sudoku hard puzzle

As a strategy we always try first—the row-column scan to find the valid cell at any stage, because that is the most basic and easiest of all techniques.

When easy breaks by row-column scan becomes hard to come by, the next technique is used.

Next easy to use technique used is—identification of single valid digit for a cell by Digit Subset Analysis or DSA in short. This technique is explained in a following concept section.

And wherever possible, Cycles are formed that in any situation are a treasure to have and Cycles play a key role in quick solution. Concept and use of Cycles are explained in a following section.

You may wait for Cycles to form automatically in a column or row.

But a proactive approach of forming a Cycle by DS analysis speeds up the solution process considerably. This is what we call forced creation of Cycles.

The last resort of filling EACH EMPTY CELL with valid digit subsets is to be taken when it is absolutely necessary. Only with all empty cells filled with valid digit subsets, the possible breakthrough points in a hard puzzle can be discovered.

Strategically for faster solution, it is better to delay this time consuming task as much as possible.

Full DS enumeration process is explained in a following section, but any experienced Sudoku player would be doing it as a routine.

In hybrid strategy, a few of the cells of interest are filled with DS of shorter length and analyzed for a breakthrough such as forming a Cycle or a single digit lockdown.

One of the most powerful patterns that we have used for highly positive result each time is the lockdown of a single digit in a row or column inside a 9 cell square so that the digit is eliminated from all other DSs in the locked row or column outside the 9 cell square.

The necessity of use of this digit lockdown technique indicates in a way the hardness of the puzzle. This technique is also explained in a following section.

In solving this Sudoku hard, we have mentioned the use of an additional technique of Parallel scanfor a single digit on the cells of a row or column.

In the case of this sudoku hard, parallel scan turns out to be one of the two possible techniques that can be used and so because of its minor role is not explained in more details.

A rarely encountered powerful pattern is 4 cell single digit lockdown in a rectangular formation that we have found only once. Naturally, it is a superset of the more common single digit lockdown in 2 cells and so is much more effective. We'll not also elaborate on this further.

A basic part of overall strategy is,

Whether we search for a breakthrough of a bottleneck or a valid cell identification, our focus usually is on the promising zones, the zones (row, column and 9 cell square combined) that contain larger number of filled digits including Cycles.

The main strategy should always be to adopt the easier and faster technique and path to the solution by looking for key patterns all the time. Digit lockdown, Cycles, Valid cell by DSA are some of the key patterns.

Focus when solving a hard Sudoku puzzle should be on using the technique that would produce best results fastest. Easy to say, not so easy to do—comes with practice.

What is a Cycle and how to use it in solving a Sudoku hard puzzle

Form of a Cycle:

In a Cycle, the digits involved are locked within the few cells forming the cycle. The locked digits can't appear in any other cell in the corresponding zone outside the few cells forming the cycle.

For example, if a 3 digit cycle (4,7,8) in column C2 is formed with a breakup of, (4,7) in R1C2, (4,7,8) in R5C2 and (7,8) in R6C2, the digits 4, 7 and 8 can't appear in any of the vacant cells in column C2 further.

If we assume 4 in R1C2, you will find R5C2 and R6C2 both to have DSs (7,8) implying either digit 7, or 8 and no other digit to occupy the two cells. This in fact is a two digit cycle in the two cells. Together with 4 in R1C2, the situation conforms to only digits 4, 7 and 8 occupying the set of three cells involved in the cycle.

Alternately if we assume 7 in R1C2 (this cell has only these two possible digit occupancy), by Digit Subset cancellation we get, digit 8 in R6C2 and digit 7 in R5C2 in that order repeating the same situation of only the digits 4,7 and 8 to occupy the set of three cells.

Effectively, the three digits involved cycle within the three cells and can't appear outside this set of three cells.

This property of a cycle limits the occupancy the cycled digits in other cells of the zone involved (which may be a row, a column or a 9 cell square). This generally simplifies the situation and occasionally provides a breakthrough by reducing the number of possible digits in the affected cells.

A number of Cycles are shown below from a Sudoku hard solution stage:

Examples of Cycles in a Sudoku game

Cycle (1,2,6) in column C1 is over all three 9 cell squares on the left. It affects only the column C1.

Cycle (3,8,9) in top right 9 cell square is also in row R2, so it should affect both the 9 cell square and R2.

But Cycle (3,6,7) in top right 9 cell square is formed only in the 9 cell squares, it affects only the cells in the 9 cell square.

Can you see another Cycle in row R1 apart from Cycle (1,6)? The second Cycle (3,6,7) is formed by the cells R1C2, R1C3 and the far away cell R1C9. This Cycle affects only the row R1.

Can you say which are the affected areas for Cycle (1,6) in R1?

Two cells of this Cycle belong to row R1 as well as to the top middle 9 cell square. So the Cycle affects two areas, the row and the 9 cell square. This will be true for any two digit Cycle.

Use of a cycle:

In the example of cycle described above, if a vacant cell R8C2 in column C2 has a possible DS of (1,4), as digit 4 has already been consumed in the cycle (4,7,8) in the column, only digit 1 can now be placed in R8C2. You get a single valid digit 1 for R8C2.

This is how a new valid cell is obtained using a Cycle that was not visible otherwise.

In any hard Sudoku game solution, creating, analyzing and using the pattern of Cycles play a very important role.

How a valid cell is identified by Digit Subset Analysis or DSA in short

Sometimes when we analyze the DSs in a cell, especially in highly occupied zones with small number of vacant cells, we find only one digit possible for placement in the cell. We call valid cell identification in this way as Digit Subset Analysis.

For example, if in row R4 we have four empty cells, R4C1, R4C3, R4C6 and R4C9 with digits left to be filled up [1,3,5,9] we say, the row R4 has a DS of [1,3,5,9] that can be analyzed for validity in each of the four empty cells.

By the occurrence of digits in other cells if we find in only cell R4C1 all the other three digits 3,5 and 9 eliminated as these are already present in the interacting zones of middle left 9 cell square and the column C1, we can say with confidence that only the left out digit 1 of the DS [1,3,5,9] can occupy the cell R4C1.

Remember that,

While evaluating the valid digit subset or DS of an empty cell, you would analyze not only the digits that are already filled in corresponding row, column and 9 cell square, you must include the Cycles present in the three interest zones also.

This is how we identify a valid cell by Digit Subset Analysis.

Identifying a valid digit in a cell by DSA is like a bread and butter technique. It is possibly the most heavily used technique after the simplest row-column scan.

Though DSA may not be considered as an advanced technique it often provides a much required breakthrough. So always look for a valid cell by DSA.

An example of a breakthrough at the late stage of Sudoku hard puzzle solution by DSA is shown below.


We'll do DSA on cell R7C5. The possible digit subset or DS in column C5 and hence in cell R7C5 is [5,7,9], but the two digits [5,9] both are present in row R7.

So eliminating these two from the three digit DS for R7C5, we get the single valid digit 7 for R7C5 --- R7C5 7.

This is a breakthrough even at this late stage.

How digits possible for all empty cells (DSs) enumerated while solving a Sudoku hard puzzle

We have not yet discussed the enumeration of every empty cell with their valid digit subsets or DSs.

Let us see this in a little detail. We'll enumerate the possible digit subset or DS for empty cell R8C1 in the following Sudoku game.

Possible digit subset DS enumeration for an empty Sudoku cell

Target cell R8C1 is colored green. Unique set of digits in the three zones—bottom left 9 cell square, row R8 and column C1 colored yellow—will determine the DS for empty cell R8C1.

To enumerate the DS for cell R8C1, look at the row R8 with six digits missing in it—1, 2, 4, 5, 6 and 7.

Now cross-scan column C1 to identify any of these six appearing in column C1.

As 5 and 7 are the two digits out of six that are missing in the intersecting row R8, cancel these two from the six digit subset for R8C1 to reduce it to [1,2,4,6]. Considering row R8 and column C1, possible digits that can occupy R8C1 till now are the DS [1,2,4,6].

But R8C1 also belongs to a 9 cell square and filled digits in it will affect the DS for the cell.

So lastly check the third dimension of the home square, the 9 cell bottom left square, for any more possible digit cancellation.

With no additional digit cancellation, the valid digit subset or DS for the cell would be four digits [1,2,4,6].

None of these four digits appear in the home square, home column or the home row for the cell R8C1.

Basically for evaluating the valid DS for a cell,

You have to cross-scan the row and column as well as check against the home square digits to identify the missing digits that may fill the cell.

This is a tedious and error-prone process.

In solving a hard Sudoku puzzle, there may be no option than to go through the full empty cell DS evaluation.

But it should be done when it has to be done and as late as possible.

Two strategic approaches are adopted to minimize the overall work load in this process—

  1. First try to find valid digits and fill the cells as much as possible using any technique so that the number of possible valid digits in empty cells as well as number of empty cells are reduced, and,
  2. Identify promising zones to evaluate the small DSs of a few cells trying for a breakthrough and so reduce the full DS evaluation load.

The second is a dynamic approach that depends on your experience and skill in identifying promising zones.

Single digit lockdown and its use in solving a Sudoku hard puzzle

Occasionally, after evaluating valid DSs for a number of empty cells, you may find that,

A single digit appears only in the DSs of two or three cells in a 9 cell square, in a column or a row, and in no other DSs in the 9 cell square.

This is what we call as single digit lockdown.

If it happens in a row (or a column) inside a 9 cell square, the digit cannot appear in any other cell in the row (or the column) outside the square.

This eliminates all occurrences of the locked digit from the DSs in the row (or the column) outside the 9 cell square. Usually it creates a much needed breakthrough. It is a very powerful pattern.

Single digit lockdown - Conditions for single digit lockdown - how to identify it

Two conditions for single digit lockdown pattern,

  1. the digit can be placed in only two or three cells of a column or a row, AND,
  2. the locking cells must also be in SAME 9 cell square.

The third desired condition is,

  • The lockdown to be effective, the locked digit should not be present as a single cell candidate in both the adjacent two 9 cell squares through which the locked column or row passes.

The following shows an example of single digit lockdown of 5 in cells R7C1 and R9C1.

sudoku single digit lock: what it is and what it does

How a single digit lockdown is formed

Look at columns C1, C2 and C3 in the bottom left 9 cell square R7R8R9-C1C2C3. Out of 3 empty cells, the cell R7C3 is debarred for placing digit 5 as column C3 has a 5 and it lights up the cell for digit 5.

5 can appear only in two cells in column C1, R7C1 and R9C1 and in no other cell in the 9 cell square or the column C1.

It is locked inside these two cells in C1 and 9 cell parent square.

How a Sudoku single digit lockdown is used - What it does

The locked digit 5 eliminates itself from the DSs of the other two empty cells R5C1 and R6C1 and a new Cycle (2,3) is created in C1.

Focus again on the bottom left 9 cell square. With Cycle (2,3) in C1, another Cycle (5,9) is formed in the two cells of the 9 cell square. As a result, digit 1 becomes the only digit left and cell R7C3 only cell left for it in the 9 cell square.

Still more happens. With 1 in C3 now, digit 9 now must occupy the cell R6C3.

These two single digit candidates obtained by the single digit lockdown of 5 affects other cells and breaks the bottleneck.

As a strategy, always form a single digit lock as soon as it is discovered.

You may think, what is the point of it, what would it achieve after all!

Well, in a similar situation in the process of solving a hard Sudoku puzzle game, the reduced DS in R9C1 formed a cycle (3,7) in column C1 and helped to pinpoint a valid digit 4 in cell R2C1 and that started a deluge of valid cell finds. This proved to be the key turning point in the whole game.

Sudoku hard technique of Double digit scan

This technique sounds simple, but being aware of its existence and identifying it would always result in an important breakthrough. This digit pattern usually occurs in very hard Sudoku.

We will explain this advanced Sudoku hard technique on the following situation in a Sudoku hard game,


Notice the two highlighted digits [1,6] appearing in both row R4 and C5. Together these two result in DIRECT FORMATION OF CYCLE (1,6) in central middle 9 cell square.

This is a double digit scan simultaneously on a row and a column.

Now observe a second set of highlighted double digits [3,9] in C5 which DIRECTLY FORMS TWO CYCLES (4,7,8) AND (3,9) IN CENTRAL MIDDLE 9 CELL SQUARE.

This is a double digit scan on a single zone of C5.

Finally, with 3 in C4, R4C4 9 and R4C6 3.

Together these two double digit scans have produced two valid cells and two Cycles. It is a major breakthrough early in the Sudou hard game.

To go through the solution of this Sudoku hard once more, clicck here.

End note on Problem solving in Sudoku

Any puzzle solving involves essentially problem solving. The general steps are,

  1. First stage analysis and breaking it down into smaller chunks if possible as well as adapt the strategy of solving this type of problem,
  2. Solving the easier component problems so that the main problem size and complexity is reduced,
  3. Detailed information collection, that is, defining the problem in more details as far as possible,
  4. Second stage analysis of structure of problem (in this case of Sudoku) and information content,
  5. Key pattern identification,
  6. Use of the key pattern to create the breakthrough,
  7. Repeating the last five steps (steps 3, 4, 5, 6 and 7) for finally solving the problem.

Though this is an extra hard Sudoku, discovering a series of three early breakthroughs, first by single digit lockdown and the next two by a new power technique of double-digit scan on one or more than one row or column simultaneously eased the problem considerably.

Forming and effective use of a large number of Cycles and DSA also played important roles in quick solution of the Sudoku hard game.

Above all, the hybrid strategic approach of evaluation of possible digits or DSs in empty cells of promising zones, identification and immediate use of result-bearing digit patterns like Cycles or Single digit lockdown speeded up the solution process considerably.

More Sudoku hard puzzles you may like to solve and learn how to solve

The updated list of Solutions to level 3, level 4 and NYTimes Sudoku hard puzzle games:

How to solve Sudoku hard puzzle games full list (includes very hard Sudoku).

Enjoy solving Sudoku hard.

By the way, Sudoku hard solution techniques are included with many of the solutions.

Enjoy also learning how to solve Sudoku hard in easy steps.