Samurai Sudoku Solver

Samurai Sudoku Solver for Five Overlapping Grids

This free Samurai Sudoku Solver is designed for the classic Gattai-5 layout: five standard 9x9 Sudoku grids arranged like a cross. There is one central Sudoku and four corner Sudokus. Each corner grid overlaps the centre grid in one shared 3x3 box, so the same nine cells must satisfy two different Sudoku grids at the same time.

That overlapping structure is what makes Samurai Sudoku harder to solve by hand and harder to check with a normal Sudoku solver. A single 9x9 solver only sees one grid. A proper Samurai solver has to track five row systems, five column systems, five sets of 3x3 boxes, and four shared overlap zones. This one does that job: enter the clues, solve the complete board, fill one cell at a time, import a puzzle string, export your work, and check whether the puzzle has a unique solution.

What a Samurai board actually looks like

The layout is hard to picture from a description and obvious as a drawing, so here is a real one, generated and then checked for how many answers it has.

Look at where the tinted boxes sit: each is the bottom-right, bottom-left, top-right or top-left corner box of one outer grid, and simultaneously a corner box of the centre. That is the whole structural difference between Samurai and five separate Sudokus, and it is the reason the puzzle cannot be pulled apart.

Samurai Sudoku Rules

Samurai Sudoku follows the ordinary rules of Sudoku inside each 9x9 grid. Every row must contain the digits 1 to 9 exactly once. Every column must contain the digits 1 to 9 exactly once. Every 3x3 box must contain the digits 1 to 9 exactly once. The difference is that there are five grids, and four of the 3x3 boxes are shared between a corner grid and the centre grid.

Those shared boxes are the heart of the puzzle. If a digit is forced in the top-left overlap, it affects both the top-left Sudoku and the central Sudoku. A mistake in one overlap can make two grids impossible at once. That is why Samurai Sudoku often feels manageable at first and then suddenly tangles in the middle. The solver keeps those relationships synchronised so you do not have to check the same overlap by hand in two separate places.

Why a normal Sudoku solver is not enough

A normal 9x9 solver can help if you isolate one of the five grids, but it cannot solve the full Samurai puzzle correctly, and the reason is worth seeing rather than taking on trust. Here is the same puzzle with one digit written into a shared box.

The blocking 6 sits in column 12, which is outside the top-left grid altogether. A solver looking only at the corner cannot see it and has no way to know the placement is wrong. Work the corner to completion on its own and you may finish it perfectly, then find the centre impossible — with nothing in the corner to tell you which of its digits caused the problem.

A dedicated Samurai solver understands the whole board. It knows which 369 cells are live, which 72 positions are empty space on the 21x21 canvas, and which cells belong to two grids. It can detect contradictions that only exist when two grids are considered together, and it can test whether the puzzle has one solution or several.

How to use the Samurai Sudoku solver

Start by copying the givens from your puzzle into the grid above. You can click a cell and type a digit, or use the number buttons. Shared cells only need to be entered once — the solver treats them as belonging to both grids automatically, so a digit placed in an overlap affects the corner grid and the centre grid together.

1. Enter every clue from your printed or online Samurai Sudoku puzzle.
2. Check the grid for obvious typing mistakes before solving.
3. Use Solve when you want the full Samurai solution, and the answer to whether it is unique.
4. Use Step to fill one cell at a time — though not the cell you might expect. See the section below.
5. Use Import if you already have a puzzle string.
6. Use Export to save or share the puzzle you entered.
7. Read the message under the grid: it reports duplicate clues, an unsolvable board, or a completed one.

If you only want to verify a finished puzzle, fill in the givens or your completed grid and let the solver check the structure. It will tell you where the first contradiction is rather than simply refusing.

Importing 369-cell and 441-cell puzzle strings

The import box accepts two formats and works out which one you gave it by counting. A 369-cell string lists only the live Samurai cells, read left to right and top to bottom. It is compact and it is what to use when your source already understands the layout. A 441-cell string represents the full 21x21 board, empty space included. Either can be written as one unbroken run of characters or as values separated by spaces, commas, semicolons or pipes. In both, use digits 1-9 for clues and 0 or a dot for blanks.

One detail matters in the 441 form and is easy to get wrong: the 72 empty positions still have to be there, and they still have to be valid characters. Use 0 or a dot for them. The parser checks all 441 values before it discards the ones it does not need, so a dash, a space-filler or a letter in an empty position is rejected even though the value would never have been used.

Export always gives you the 369-cell active form, written as one unbroken string with 0 for each blank. That is the version to keep, and it re-imports unchanged. Exporting is worth doing even when you typed the board in yourself, because it gives you a clean copy to save, to send to somebody, or to compare against your source.

There is also a way to link straight to a puzzle, which the page has never mentioned: add ?puzzle= and the puzzle string to this page’s address, and the board is loaded and validated as soon as the page opens. Either length works. That is the tidy way to send someone a specific Samurai, or to bookmark the one you are working through.

What the solver checks before it searches

Before solving, the tool validates what you have entered: every live cell holds a digit from 1 to 9 or nothing, and no row, column or 3x3 box of any of the five grids repeats a digit. Because the shared boxes are checked as part of both grids they belong to, a clue that breaks only the centre is caught even when the corner it sits in looks fine. Empty positions are never treated as part of the puzzle.

That check looks for repeats, and nothing else. A board can pass it cleanly and still have no solution, which the search discovers afterwards. So "no solution" and "duplicate 6 in row 8" are different messages: the second points at a cell, while the first only says the whole search came back empty-handed, and on 369 cells the cause can be a long way from where you notice it.

When to use Step instead of Solve

Step is the button for when you want help without giving the whole puzzle away, and it is worth knowing exactly what it does, because it is not what the name suggests. Step solves the entire board in the background first, then fills the next empty cell in reading order — left to right, top to bottom across the 21x21 layout — copying from the answer it already holds. It names the row, the column and the digit, and says nothing about why.

So it is not choosing an instructive placement, and there is no deduction behind its choice to go looking for. The cell it fills is simply the first empty one on the board. What Step is genuinely good for is getting unstuck: reveal one cell, then work out for yourself which row, column, 3x3 box — or which of the two grids, if it landed in an overlap — forced it. On a board this size that is a real service, because the hard part is often just finding somewhere to resume.

Solve is the better choice when you want an answer key or a quick check. It is also the only route to the uniqueness result: after it finds a completed board it goes back to your original clues and searches for a second answer, then reports one solution or several. Stepping through to the end never produces that message, because the check belongs to Solve alone.

Common Samurai Sudoku mistakes

The most common mistake is entering a clue in the wrong copy of an overlapping box. The overlap is shared, so there is no separate version for the corner grid and the centre grid. Another frequent problem is importing a 441-position string into a system expecting 369 live cells, or the other way round. The board may look almost right and be shifted in the later rows.

Players also sometimes assume each 9x9 grid can be solved independently. In easier Samurai puzzles a corner may make progress on its own for a while. In harder ones a corner might need information from the centre, and the centre might need information from a different corner. The whole point of the format is that logic travels through the shared boxes — which is exactly what the second diagram above shows going wrong.

Strategies for solving Samurai Sudoku by hand

Begin with the four corner grids. They usually have more ordinary Sudoku structure available before the overlaps become difficult. Fill obvious singles and mark candidates in the shared 3x3 boxes. Then move to the centre grid and see how those overlap candidates restrict central rows, columns and boxes. Repeat that loop rather than trying to finish one grid completely before looking at the others.

Candidate notation is especially helpful. Write small digits in empty cells and be disciplined about removing candidates when a digit appears in a shared row, column or box. Look for naked singles, hidden singles, naked pairs, pointing pairs and box-line reduction. These classic techniques all still work, and the best opportunities often appear where a candidate is restricted by two grids at once.

If you get stuck, use the solver as a diagnostic rather than an answer. Enter the givens and compare its next placement with your pencil marks. If your candidates disagree with it, check the overlap boxes first. A single mistaken candidate in a shared region can block progress across a surprisingly large part of the board.

What to do if the solver finds a problem

If the solver reports that a Samurai Sudoku is invalid, start by checking the givens rather than assuming the published puzzle is broken. Confirm every clue in the four shared 3x3 boxes, then check whether a digit has been typed into the wrong cell of a corner grid. Imported strings deserve the same care: make sure you used the right format, because a 369-cell live string and a 441-position full-board string are not interchangeable.

If the solver says a puzzle has more than one solution, the clues are insufficient. That happens with hand-made puzzles and with grids copied from images where a given was missed. Add the missing clue if you can identify it, or treat the result as a warning that the puzzle may not be logically fair. A well-made Samurai should force a single final board without guessing.

Good uses, privacy and browser

The solver is free, runs entirely in your browser and needs no account. Nothing is stored between visits, so if you want to keep a board, export it before you leave. It is useful for checking a Samurai from a newspaper, validating one from a book, finishing a difficult online grid, turning a puzzle into a shareable string, or confirming that a puzzle you created has exactly one solution. Teachers and puzzle club organisers can check handouts before printing.

If you would rather play than check, the Samurai Sudoku page has puzzles to solve, and printable Samurai Sudoku has them on paper. For ordinary grids the 9x9 Sudoku Solver is simpler and faster, and the Sudoku Helper is a useful companion when you want the reasoning rather than the answer.