Windoku – Hard

Windoku

Windoku is a variation of Sudoku based on the classic 9x9 Sudoku. The regular Sudoku rules still apply: each row, each column, and each normal 3x3 block must contain the numbers 1 through 9 exactly once. Additionally, there are four marked 3x3 regions inside the grid. Each of these four extra regions must also contain the numbers 1 through 9 exactly once.

These extra regions are often called windows. Therefore, Windoku is also known as Window Sudoku. Other common names are Hyper Sudoku or NRC Sudoku. The basic idea remains the same as with classic Sudoku, but each marked window provides an additional constraint. Many safe steps arise not only from rows, columns, and normal blocks but especially from these four windows.

Basic rules

• The playing field consists of 9 rows and 9 columns.
• Exactly one number from 1 to 9 is entered into each empty cell.
• Each number from 1 to 9 must occur exactly once in each row.
• Each number from 1 to 9 must occur exactly once in each column.
• Each normal 3x3 block must contain each number from 1 to 9 exactly once.
• There are also four marked 3x3 windows inside the grid.
• Each of these four windows must also contain each number from 1 to 9 exactly once.
• The additional windows do not replace the normal 3x3 blocks. Both types of regions are valid simultaneously.
• A cell in a window belongs to four regions: row, column, normal 3x3 block, and Windoku window.
• The given numbers may not be changed.
• A Windoku is solved when all cells are filled and all rows, columns, normal 3x3 blocks, and all four additional windows contain the numbers 1 to 9 exactly once.

Strategies for solving

Windoku is fundamentally solved like a normal Sudoku. You look for safe entries by excluding numbers. The key difference is: the four additional windows provide further exclusions. As a result, a cell can be forbidden even if it still seems possible according to row, column, and normal block.

1. First check the additional windows

A good starting point is to look for nearly complete windows. A Windoku window functions like an additional 3x3 region. If only one number is missing, it can be entered immediately.

In the following grid, we consider the top-left Windoku window. It consists of rows 2 to 4 and columns 2 to 4.

In the top-left window, the numbers 1, 2, 4, 5, 6, 7, 8, and 9 are already present. The only missing number is 3. Therefore, the empty cell in row 4, column 2 inside this window must be a 3.

This conclusion does not arise from the normal top-left 3x3 block, but from the additional Windoku window. Exactly such steps are typical for this variant.

2. A window can trigger a follow-up step in a row

Often, a window not only solves a single cell but also directly prepares the next Sudoku step. In the following example, we consider the top-right Windoku window. It consists of rows 2 to 4 and columns 6 to 8.

In the top-right window, the numbers 1, 3, 4, 5, 6, 7, 8, and 9 are already present. The only missing number is 2. The empty cell in row 2, column 6 inside this window must therefore be a 2.

Now row 2 is nearly complete. It now contains 1, 2, 3, 5, 6, 7, 8, and 9. The only missing number is 4. So, the last empty cell in row 2, column 9, must be a 4.

Thus, the second entry arises as a direct consequence of the window rule. This is an important Windoku learning point: an extra window can assign a number, and this number can then help progress in a normal row, column, or block.

Typical solving process

1. First check the regular Sudoku rules: rows, columns, and 3x3 blocks.
2. Mark or note the four Windoku windows additionally.
3. Look for missing numbers in the windows, especially when many numbers are already present.
4. Always check each window cell with four conditions: row, column, normal block, and window.
5. Use window entries as triggers for follow-up steps in rows, columns, and normal blocks.
6. Compare windows with normal 3x3 blocks, as they overlap in some areas.
7. Note candidates when no immediately safe entries are visible.
8. Eliminate candidates consistently, even if they are excluded only by a Windoku window.
9. Regularly check that no number appears twice in any window.

Common mistakes

• Forget the additional windows: Windoku is not just a normal Sudoku with decoration. The marked 3x3 windows are real extra regions.
• Confusing windows with normal blocks: The four windows do not replace the normal 3x3 blocks. Both are valid at the same time.
• Checking a window cell with only three regions: For window cells, all four regions must always be checked.
• Placing a number twice in a window: Even if row, column, and normal block fit, the number can be forbidden by the window.
• Rushing too early: Like in classic Sudoku, only logically safe numbers should be entered.

Tips for beginners

• Treat the four windows like additional 3x3 blocks.
• Always check row, column, normal block, and window for each empty window cell.
• First look for windows where many numbers are already present.
• If stuck, go through numbers one by one in the windows and ask: Where can this number still be in this cell?
• Use candidate notes especially in window cells because an additional constraint applies there.
• Check after each entry whether the number is still allowed in the associated window.
• Pay attention to follow-up steps: a safe entry in the window can immediately advance progress in a row or column.

Windoku extends the classic Sudoku by four additional 3x3 regions. This creates more constraints but also more starting points. Those who treat the windows consistently as separate regions and combine them with rows, columns, and normal blocks can solve Windoku logically and without guessing.