Sudoku 10×10 – Hard

Sudoku 10x10

Sudoku 10x10 is a larger variation of classic Sudoku. Instead of a 9x9 grid, the puzzle consists of 10 rows and 10 columns. As a result, there are not only nine but ten different symbols. In this variant, the characters 1 to 9 and A are typically used. The A simply represents the tenth symbol and is not read as a letter with special significance.

The goal remains the same as in classic Sudoku: all empty cells must be filled so that each symbol appears exactly once in each row, column, and region. In Sudoku 10x10, the regions are typically rectangular blocks. In the format used here, each region consists of 2 rows and 5 columns.

Sudoku 10x10 is not a mathematical calculation. The characters 1 to 9 and A are not added or calculated. They serve only as distinguishable symbols. It is only important where a symbol is allowed to stand and where it is excluded by the rules.

Basic Rules

• The playing field consists of 10 rows and 10 columns.
• Ten different symbols are used: 1, 2, 3, 4, 5, 6, 7, 8, 9, and A.
• A is simply the tenth symbol. It has no arithmetic significance.
• The playing field is additionally divided into regions. In this 10x10 variation, the regions consist of 2 rows and 5 columns.
• Exactly one of the ten symbols is entered in each empty cell.
• Each symbol must appear exactly once in each row.
• Each symbol must appear exactly once in each column.
• Each 2x5 region must contain each symbol exactly once.
• The given symbols must not be changed.
• A Sudoku 10x10 is solved when all cells are filled and no row, column, or region contains a duplicate symbol.

Strategies for Solving

The most important ways of thinking are the same as in classic Sudoku: You look for certain entries and exclusions. A number or symbol can only be entered when it is logically certain that no other possibility remains.

1. Starting with almost complete regions

A good start is rows, columns, or regions that already have many symbols. The more symbols are present, the fewer options remain.

In the first row, symbols 1 to 9 are already present. The only missing symbol is A. Therefore, an A must be in the last cell of the first row.

This conclusion is clear because only one symbol is missing in the row.

2. Single candidate: When a cell has only one possibility

A cell in Sudoku 10x10 always belongs to three areas: its row, its column, and its 2x5 region. All symbols already appearing in these areas are excluded for this cell.

In the following example, we examine the top-left cell.

In the first row, the symbols 2, 3, and A are missing. But in the first column, 2 and A are already present. This leaves only the symbol 3 possible for the top-left cell.

So, the 3 is not guessed but is the only possibility that fits the row, column, and region.

3. Hidden single candidate: When a symbol only fits in one position

Sometimes a empty cell has multiple possible symbols. Still, a particular symbol can only be in one place in a row, column, or region. Then, this symbol is definitely placed there.

In the following example, we look for the position of the 5 in the upper-left 2x5 region.

The top-left region consists of the cells in the first two rows and the first five columns. In row 2, a 5 is already there, so it cannot be in row 2 of this region. Moreover, columns 1, 2, 4, and 5 already contain 5s. This further excludes multiple cells in the first row of this region.

Therefore, only the cell in row 1, column 3 remains for the 5 in this region.

This is a hidden single candidate. The cell is not initially obvious; instead, the symbol 5 had only one possible position in this region.

4. Tracking a symbol through the grid

A very useful method is to track a single symbol across the entire grid. For example, selecting the 7 and observing which rows, columns, and regions are already blocked by 7s.

In the following example, we examine the top-right 2x5 region, i.e., the first two rows and the last five columns.

The 7 in the first row excludes all other 7s in that row. Therefore, the 7 cannot be in row 1 of the top-right region. In row 2, within that region, columns 6 to 10 are left. But with existing 7s in columns 6, 7, 8, and 9, four of these five cells are excluded.

This leaves only row 2, column 10 for the 7 in this region.

This technique is especially helpful because you don’t have to check each cell individually. You follow a symbol and use existing entries as exclusions.

5. Noting candidates

In more difficult Sudoku 10x10 puzzles, it is often not enough to immediately enter new symbols. It helps to note possible symbols as candidates. A candidate is a symbol still permitted in a cell based on current information.

6. Checking all three conditions

An entry is correct only if it fits the row, column, and region simultaneously. Especially in 10x10 Sudoku, it's easy to overlook a symbol because more characters are involved than in classic 9x9 Sudoku.

Before each entry, always verify three questions:

• Does this symbol already appear in the row?
• Does this symbol already occur in the column?
• Does this symbol already exist in the 2x5 region?

Only if all three checks are passed and no other possibility remains, is the entry secure.

Typical Solution Process

It’s best not to solve a 10x10 Sudoku strictly from top-left to bottom-right. It’s more effective to repeatedly look for areas with many clues and symbols with strong restrictions.

1. First, check rows, columns, and 2x5 regions with many given symbols.
2. Look for areas where only a few symbols are missing.
3. Check individual cells by combining row, column, and region.
4. Search for single candidates: cells where only one symbol is possible.
5. Find hidden single candidates: symbols that can only be in one position within a region.
6. Follow individual symbols across the grid.
7. Note candidates when no immediate secure entries are visible.
8. Update candidates after each new entry.
9. Search for candidate pairs or similar patterns if simple steps are no longer enough.
10. Regularly check that no row, column, or region contains a duplicate symbol.

Common Mistakes

• Rushing too early: A good solution is based on secure exclusions, not guesses.
• Not updating candidates: After each new entry, candidates in row, column, and region must be eliminated.
• Focusing on only one cell: The solution often results from combining multiple areas.
• Overlooking hidden single candidates: A symbol can be certain even if the cell seems to have multiple candidates.

Tips for Beginners

• First, get familiar with the ten symbols 1 to 9 and A. Treat A simply as another number.
• Start by looking for rows, columns, or regions with many clues.
• Always check row, column, and 2x5 region for each cell.
• Track individual symbols across the entire grid, for example, first all possible positions for 1, then for 2, and so on.
• Use candidate notes earlier than in classic Sudoku because there are more options in 10x10.
• Eliminate candidates only if the exclusion is truly justified by a rule.
• Ask yourself before each entry: Why must this symbol be here exactly?
• If you get stuck, change your perspective: from rows to columns, from columns to regions, or from cells to individual symbols.
• Keep the grid tidy. With ten symbols, cluttered notes quickly lead to mistakes.
• Focus first on secure entries. Hard patterns become important only when simple single candidates and hidden single candidates are exhausted.

Sudoku 10x10 is a larger and somewhat more challenging form of classic Sudoku. The additional symbols and the 2x5 regions make the puzzle more complex, but the fundamental idea remains: through careful exclusions, you find secure entries step by step until the entire grid is solved.